Before you buy any used RV, treat the propane (LP) system as a safety system, not a feature. The two things that matter most are simple: the gas itself must not leak, and the propane detector must work. A healthy LP system on a used rig is usually a few hundred dollars to make right; an ignored one is how people get carbon monoxide poisoning, refrigerator fires, and tanks that no station will legally fill. Test the detector on the walkthrough, and budget to replace it if it is older than about 5 to 7 years.
Propane runs the most important comfort systems in almost every RV: the furnace, the water heater, the cooktop, and (on absorption models) the refrigerator. It is safe when the system is sound, and it is unforgiving when it is not. The good news for a buyer is that propane problems are some of the easiest to catch on a walkthrough if you know exactly where to look and what to smell, hear, and read. This guide walks you through it like a friend in the passenger seat.
How the RV propane system actually works
You do not need to be a gas tech, but you should understand the four parts so you can inspect each one.
- The tanks or cylinders. Travel trailers and fifth wheels use removable DOT cylinders (usually two 20 lb or 30 lb bottles up front). Motorhomes (Class A and most Class C) use a fixed ASME tank mounted to the frame. The type matters for inspection and recertification, which we cover below.
- The regulator. This is the small dome (often with a cover) that drops tank pressure down to the low pressure the appliances need. A failing regulator causes weak flames, sooting, and appliances that will not light.
- The lines. Copper and rubber hoses carry gas from the tank to each appliance. Age, rodents, and vibration are the enemies here.
- The appliances and the detector. Each appliance has its own safety shutoff, and the cabin has an LP gas detector mounted low (propane is heavier than air and pools near the floor).
Different rig types put these parts in different places and stress them differently. A travel trailer or fifth wheel carries exposed front cylinders that take road spray and UV. A Class A motorhome or Class C hides a fixed tank under the coach where you have to crawl to see it. A camper van often has a single small cylinder in a sealed compartment. Know which you are dealing with before you go look.
The propane detector: your single most important check
The LP detector (and often a combined LP/CO detector) is the cheapest life-safety device in the rig, and it is the one most likely to be dead. These detectors have a fixed lifespan because the sensor degrades whether the rig is used or not.
| Detector fact | Typical reality | Why it matters to you |
|---|---|---|
| Lifespan | 5 to 7 years from the manufacture date stamped on the unit | After that the sensor drifts and may not alarm on a real leak |
| Manufacture date | Printed on the face or back label | A 2014-dated detector in a 2024 sale is long expired |
| Replacement cost | $25 to $60 for the detector, $80 to $150 if a tech wires it | 12V hardwired units are not a battery swap |
| Power | Usually hardwired to 12V house power, not a 9V battery | It runs whenever the house battery is connected |
| Alarm test | A test button checks the circuit, not always the sensor | A passing test does not prove the sensor still senses gas |
On the walkthrough, do this: find the detector (low on a wall near the floor, often near the kitchen or the bed). Read the date on it out loud. Press the test button with the house power on and confirm it chirps or sounds. If you cannot find a date, or the date is more than 5 years back, plan to replace it and treat the seller's "it works" as unverified.
Verify the exact age and type on the specific unit, because a combined LP/CO detector and a propane-only detector look similar and have different costs. Replacing one yourself is a 20-minute job if it is the plug-in type; budget the higher figure if it is hardwired behind a panel.
Walking the LP system: a buyer's checklist
Here is the full sequence to run on a used rig, in order. Do it with the seller present and ask them to operate the tank valves so you are not the one touching their property.
- Read the tank dates. DOT cylinders are stamped with a manufacture date (month and year). They must be recertified 12 years after manufacture, then every 5 years after that. A cylinder past its date is legal to own but no station will refill it.
- Check the ASME tank (motorhomes). Fixed tanks do not expire the same way but inspect for heavy rust, dents, and a working gauge. Verify the date and condition on the specific unit.
- Look at the regulator. Confirm it has a cover and is not cracked, corroded green, or hanging loose. A two-stage regulator with an indicator window is a plus.
- Inspect the hoses and lines. Rubber pigtail hoses crack and dry out; copper lines should be secured, not rubbing on a frame edge. Look for green corrosion at fittings (a sign of a slow leak).
- Sniff test. With tanks on, smell around the tanks, the regulator, and inside near the appliances. Propane is odorized to smell like rotten eggs. Any persistent smell is a walk-away until fixed.
- Soap test the fittings (if the seller allows). A spray bottle of soapy water on a fitting will bubble at a leak. Many sellers will not want this; a pre-purchase inspection will do it properly.
- Light every appliance. Cooktop should light with a steady blue flame. Furnace should ignite and blow warm. Water heater should fire. Fridge on gas mode should cool (this can take hours, so check the flame lights at minimum).
- Watch the flame color. Steady blue is good. Yellow, lazy, or sooting flames point to a regulator, a venturi blockage (often spider nests), or a pressure problem.
- Confirm the detector. Date, test button, and that it powers up with the house battery.
If three or more of these fail, the LP system needs a professional service before you trust it, and that is leverage on price.
Tank age, recertification, and refusal at the pump
This trips up first-time buyers constantly. A DOT cylinder that looks fine can still be refused at every propane station because it is out of date.
The rule of thumb for removable cylinders: usable for 12 years from the manufacture date, then recertify every 5 years after that. Recertification is cheap (often $5 to $20 per cylinder at a propane dealer) but it requires the dealer to inspect and re-stamp the bottle. If the date is buried, a dealer can still recertify a sound tank; a rusted or damaged one gets condemned.
| Cylinder situation | What it costs you | Action |
|---|---|---|
| In-date 20 lb steel cylinder | $0 | Use as is, note the next due date |
| Out-of-date but sound | $5 to $20 recert per tank | Recertify, no replacement needed |
| Rusted, dented, or condemned | $45 to $90 to replace each | Budget replacement |
| Empty at sale (cannot test appliances) | Hidden risk | Fill before closing or discount |
Seller question to ask: "Are the tanks in date, and can we fill them before I commit so I can test the furnace and fridge?" An empty tank at sale is not automatically a red flag, but it conveniently prevents you from testing the most expensive propane appliances. Treat untestable appliances as unverified and price accordingly. You can fold these numbers into your real cost-of-ownership math with the ownership cost calculator and turn a failed test into a lower number with the used RV offer calculator.
Detecting and judging a leak
There are three ways a leak shows up, and you want to recognize all three.
- Smell. The rotten-egg odorant is added precisely so you notice. A faint whiff that clears is one thing; a persistent smell near the tanks or a fitting is a stop sign.
- Bubbles. Soapy water on a fitting bubbling steadily is a confirmed leak. This is the gold-standard field test.
- Pressure drop. A professional does a timed pressure test (a manometer holds gas pressure and watches for a drop over several minutes). This catches slow leaks you cannot smell. A pre-purchase RV inspection (typically $150 to $500 depending on depth) almost always includes this, and on a propane-heavy rig it is worth it.
Soot and yellow flames are an indirect warning. They mean incomplete combustion, which on a vented appliance is inefficiency, but on a fridge or water heater can mean a venting or pressure issue worth a tech's eyes. Combustion problems also tie into electrical gremlins when igniters and control boards are involved, so a rig with sketchy 12V power often has flaky propane ignition too.
What propane repairs typically cost
So you can negotiate from real numbers, here is the honest range. Verify against the specific unit and your local labor rates.
| Repair | Typical cost range | Notes |
|---|---|---|
| Replace LP/CO detector | $80 to $150 installed | DIY for the plug-in type at $25 to $60 |
| Recertify DOT cylinders | $5 to $20 each | Required past the date stamp |
| Replace a 20 to 30 lb cylinder | $45 to $90 each | If condemned or rusted |
| New two-stage regulator | $40 to $120 plus labor | Fixes weak or yellow flames |
| Replace rubber pigtail hoses | $20 to $50 each | Cheap insurance on an old rig |
| Pressure test and leak repair | $100 to $300 | Includes finding and fixing the leak |
| Furnace gas valve or board | $150 to $500 | Where propane meets electrical problems |
| Fridge cooling unit (gas absorption) | $800 to $2,000+ | The expensive surprise; test the fridge |
The pattern: detectors, hoses, regulators, and recerts are all small money. The thing that can hurt is an absorption refrigerator that will not cool on gas, because the cooling unit is a major repair or a full fridge replacement. That is why lighting the fridge on the walkthrough matters even if you cannot wait hours to confirm it gets cold.
Walk-away conditions
Most propane issues are fixable and become price leverage. A few should make you pause hard or walk.
- A persistent gas smell the seller cannot explain or fix. Active leaks are not a "later" item.
- A dead detector plus an old, soot-stained appliance. That combination suggests the safety systems were ignored, and where one was ignored, others usually were.
- A condemned tank with appliances that will not light and an unwilling seller. If you cannot test the system at all and the seller blocks an inspection, you are buying blind.
- Evidence of a prior propane fire (scorching near the fridge vent, a replaced exterior panel). Combine this check with your scan for RV red flags and the full used RV inspection checklist.
None of these is automatically the end of the deal, but each one moves the rig toward "have a professional inspect before money changes hands." When you paste the listing in to get your RV verdict, flag any mention of propane work, a new fridge, or "needs furnace serviced," because those notes tell you where the system has already been hurting.
FAQ
How long do RV propane detectors last? Typically 5 to 7 years from the manufacture date printed on the unit, regardless of how much the RV was used. The sensor degrades on a timer. Read the date on the specific detector and replace it if it is older than 5 years; a fresh one is $25 to $60 for the part.
Is it normal for a used RV to smell faintly of propane? A brief whiff when you first open the tank valves can happen, but it should clear quickly. A persistent rotten-egg smell near the tanks, regulator, or appliances means an active leak and is a walk-away until a tech finds and fixes it.
Can I refill any propane tank that came with the RV? Only if the DOT cylinder is in date. Removable cylinders are good for 12 years from manufacture, then must be recertified every 5 years. Stations refuse out-of-date tanks. Recert is cheap ($5 to $20); replacement runs $45 to $90 if the tank is condemned.
Should I pay for a professional propane pressure test before buying? On a propane-heavy rig (furnace, water heater, and gas fridge), yes. A pre-purchase inspection with a timed pressure test ($150 to $500 total) catches slow leaks you cannot smell and confirms the appliances actually run on gas.
The fridge runs on electric but I cannot tell if it works on propane. Does it matter? It matters a lot. Absorption fridges are the expensive propane appliance, and a failed gas cooling unit is $800 to $2,000-plus. Confirm the fridge at least lights on gas mode during the walkthrough, even if you cannot wait hours to verify it gets cold.
Paste the listing and get your RV verdict before you trust a single word about the propane system.