Thanksgiving Dry Camping: 47 lb of Propane in Four Cold Nights

Four nights in a relative’s driveway at an average of 30 degrees burns 11 gallons of propane. That is 47 lb of gas, and 41 lb of it comes off the trailer ball, because the cylinders sit on the A-frame where 87 percent of their weight reaches the coupler.

The propane is the arithmetic nobody argues about. The furnace blower is the part that ruins the trip: the same four nights pull 156 amp hours out of a battery that holds 45 usable ones, and whether that matters depends entirely on one question you should ask before you accept the invitation.

91,500 BTU a Gallon, Then a Duty Cycle

A gallon of liquid propane holds about 91,500 BTU, and it weighs 4.24 lb. Those two numbers turn any furnace nameplate into a fuel plan. A 30,000 BTU furnace running flat out drinks 0.328 gallons an hour, which is 1.39 lb, and it does that only on the coldest night of the year.

The rest of the time it cycles. Duty cycle is the share of the trip the burner is actually lit, and it moves faster than the thermometer does, because wind and a poorly sealed slideout matter as much as the outside temperature. Four nights is 96 hours, day and night, which is the honest denominator for a trip where the trailer sits parked the whole time.

30,000 BTU input, 91,500 BTU per gallon, 4.24 lb per gallon, 96 hours of trip time, two 30 lb cylinders holding 60 lb of gas between them. Duty cycle is burner-on time as a share of total hours, not of night hours alone. Assumes a thermostat set near 65 degrees and no electric space heater running alongside.

Most Thanksgivings land on the 30 degree row, and 47 lb is 78 percent of a 60 lb pair. You pull out with full cylinders and come home with 13 lb. That is enough for the drive and nothing else, which is why the refill belongs on Wednesday’s list and not Friday’s.

Cold cylinders add a second problem the table cannot show. A cylinder that will feed a 100,000 BTU load at 60 degrees is down to roughly 35,000 BTU an hour at 20 degrees, and under 25,000 near zero. A 30,000 BTU furnace running near continuous at that temperature is asking for more gas than one cold 30 lb cylinder can boil off, so the flame gets short and the burner drops out on low pressure before the tank is empty.

Two Full 30s Put 98 lb on the Ball

A full 30 lb cylinder is 56 lb on a scale: 30 lb of gas and about 26 lb of steel. Two of them are 112 lb, and on a travel trailer that weight lives on the A-frame, ahead of the axle, where it does the most damage to your payload budget.

It is still a lever, and the lever is worth running. On this 24 ft trailer the coupler-to-axle span is 234 in, and the cylinder tray sits about 30 in behind the coupler, so the cylinders are 204 in ahead of the axle center. Their share of coupler load is 204 divided by 234, or 87 percent. Two full 30 lb cylinders put 98 lb on the ball and 14 lb down through the axles.

Now the part almost nobody accounts for. RVIA shipping weight is figured with the propane cylinders full and the water tanks empty, so the pair the trailer shipped with is already inside the 818 lb dry tongue on the sticker. If it shipped with two 20 lb cylinders, that pair is 74 lb and 65 lb of it is on the ball. Swapping to 30s adds 38 lb to the trailer and 33 lb to the coupler, which moves tongue weight from 12.6 to about 13.0 percent. Still inside the window. Still weight that came out of nothing.

A third cylinder for a cold week is 56 lb more, 49 lb of it on the ball, and now the tongue is at 13.8 percent with a full water tank behind the axle pulling it back down. That combination is how a trailer that towed fine in July starts to feel light on the front axle in November.

Cylinder Setups, Four Nights at 30°F

The cylinder choice is the only decision in this article that changes both the fuel plan and the coupler load at the same time, so it is worth putting four options side by side.

Gas weight is the propane alone; full weight adds the steel. Tare weights used here are 17 lb for a 20 lb cylinder, 26 lb for a 30 and 32 lb for a 40. Your cylinder has its tare stamped on the collar as TW, and the stamped number beats this table. Coupler share is 204 divided by 234 of total cylinder weight. Nights are 12 hour nights at a 35 percent duty cycle.

The 20 lb pair does not finish four nights. It gets through three and part of a fourth, and the fourth is the night before the drive home, when the trailer is parked and nobody wants to move it. The 30 lb pair finishes with 13 lb, which is thin. The spare cylinder is the option that actually removes the risk, and it costs 49 lb of coupler load and a strap.

One Extension Cord Decides the Battery Question

The furnace is a 12 volt appliance with a propane flame. Its blower pulls about 7.5 amps while it runs, and at a 35 percent duty cycle across a twelve hour night that is 31 amp hours, plus another 8 for lights, the control board and the fridge board. Call it 39 amp hours a night and 156 for the trip.

A single group 27 flooded battery is 90 amp hours, and you can spend half of that without shortening its life, so 45 usable. That is one night and part of a second. A pair of 6 volt golf cart batteries in series is 215 amp hours and 107 usable, which gets you to the middle of night three. If the driveway has no live outlet, this is the constraint that ends the trip early, not the propane.

If the outlet is live, the math flips. A 15 amp household circuit gives you about 12 amps continuous, so a 55 amp converter can push roughly 30 amps of DC into the battery as long as nothing else is drawing. That replaces a night of furnace draw in about eighty minutes. Ask two questions when you accept the invitation: is there an outdoor outlet, and is anything else on that circuit. A garage freezer that cycles at 2 a.m. plus a converter at full output is a tripped breaker and a cold trailer.

Without an outlet, the fallback is a generator in the truck bed at about 95 lb, which counts against payload in both directions and sits behind the rear axle where a half-ton feels it most. Against a 1,480 lb payload sticker that is 6.4 percent before a single passenger gets in. The other fallback is a roof panel, and late November at 40 degrees north gives you roughly two equivalent sun hours, so 200 watts returns about 30 amp hours a day. That holds the line. It does not recover a deficit.

What Comes Back on the Ball

The drive home is 41 lb lighter at the coupler than the drive out, because the gas that boiled off the A-frame is gone and the steel stayed. That sounds like good news and it is not quite. The gray tank is the counterweight: four nights of dishes, showers and coffee is 30 gallons, or 250 lb, and where it sits decides whether the 41 lb you lost matters.

On a rear-bath floorplan the gray tank is behind the axles, so 250 lb back there takes 43 lb off the ball. Add that to the 41 lb of burned propane and a trailer that left at 12.6 percent comes home near 11.6, with the water still sloshing. The return leg tongue weight guide covers that side of the trip, and it is the reason to dump the gray tank at the other end rather than haul it home.

The Five Checks Before You Hitch

Four of these take ten minutes. The fifth one is the reason a trailer arrives at Thanksgiving with a full cylinder and a dead battery.

1. 1

   Read the furnace nameplate, not the thermostat

   The input rating is stamped on a plate behind the louver, usually 20,000 to 40,000 BTU. Add the water heater if you plan to shower, and leave the fridge out of it because the absorption flame is small. Whatever number you find is the full-fire rate for every appliance downstream of the regulator.

2. 2

   Divide by 91,500, then multiply by the duty cycle

   A 30,000 BTU furnace is 0.328 gallons an hour at full fire. Your duty cycle is not on any label, but it tracks the overnight low: 20 percent in the forties, 35 percent around freezing, 55 percent in the low twenties, 70 percent below fifteen. Multiply the two and you have gallons an hour for planning.

3. 3

   Check the manufacture date on the cylinder collar

   The date is stamped in the steel, not on a sticker. A DOT cylinder needs requalification twelve years after manufacture and every five years after that. A cylinder that fails that rule is not a spare, it is a cylinder the exchange counter will refuse, and you find out at the counter on Wednesday night.

4. 4

   Weigh the tongue with the cylinders full

   Full is the state you leave in, so it is the number that matters. Two full 30 lb cylinders put 98 lb on the coupler on a 24 ft trailer, and the tank of water behind the axle has already taken some of that back. Put the loaded number in the tongue weight calculator rather than assuming the dry sticker still applies.

5. 5

   Test the system for leaks before the trip, not after

   Propane manifold pressure should sit at 11 inches of water column, which is about 0.4 psi, and a manometer is the only gauge that reads in that unit. With the appliances off and one cylinder open, a falling column means a joint is passing gas. Four nights in someone's driveway is a bad time to learn that the flare at the furnace valve was never tight.

One more item for the Wednesday run: exchange or refill the cylinders before the holiday, not during it. Most exchange cages are unattended on Thanksgiving Day, the hardware stores that do refills open late on Black Friday, and a propane run on Thursday morning costs you the only quiet hour of the weekend. The cold weather towing guide covers what else changes below freezing, and the winterizing walkthrough is the follow-on if this is the last trip of the season.

Sources & References

• Propane Education & Research Council: energy content, cylinder sizing and cold weather vaporization
• NFPA 1192: recreational vehicle plumbing, propane systems and appliance venting
• PHMSA: DOT cylinder qualification and requalification intervals
• RV Industry Association: shipping weight, cargo carrying capacity and propane in the weight label
• National Weather Service: overnight low and wind forecasts for furnace duty cycle planning