Many truck campers prefer to have their propane tank refilled as opposed to exchanging them.
Refilling the propane is a better value than exchanging them at a big box store or truck stop. Refilling the tank so that you are not tied to a single supplier.
The exchanged tanks are generally filled with only about 15 lbs., 3.5 gallons of propane rather than the maximum limit of 20 lbs. A standard 20 lb. refilled tank will weigh 19.9 lb., about 4.7 gallons of propane.
Standard truck camper propane tanks are meant to be filled to no more than 80% capacity. A valve at the top of the cylinder called “overfill protection device” (OPD) is required on all cylinders to prevent overfilling. The 20% is called “ullage” and allows for expansion during temperature swings which play a major role. The risk of overfilling is complicated because the liquid in the tank will expand in warmer weather as the pressure in the tank increases from about 60 psi at 30 degrees F to about 200 psi at 100 degrees F. The tank may also cause the relief valve to open and spill liquid propane onto the ground causing damage, injury, and even a fire.