A 20lb. propane tank is designed to safely hold 4.6 gallons of liquid propane (20 lbs.) There is sufficient head room for expansion designed into the tank. If the tank cannot be filled with 20 lbs of fuel, either the OPD is not adjusted correctly or the operator is not filling the tank to capacity.
I have my tanks filled at a welding supply company and have a wholesale account with them. I get my tanks filled for 10 bucks, retail customers pay $18. The tare weight on my 20# tanks is 18 lbs, so my full tanks are 38 lbs. full. Another gas station down the street charges 15 bucks to “fill” your tank, and he only puts about 15 lbs of fuel in.
I also have a little 5lb. vertical tank in the truck just in case the 20lb tank runs out.