Allot of refill stations around me have a minimum 8 gallons of
propane. They either don't want the hassle of filling a single bottle or will skim you for the extra gallons when the bottle is full. I carry 2 full bottles on my trailer all the time. I have never used more than 1 bottle on any trip. (we are still working so no long trips for us) Once the bottle has been used any I place it on my BBQ until empty. I put a full one on the trailer for the next trip.
I have found trading my empty bottle at Home Depot works well for me. I have 4 bottles, 2 for my trailer and 2 for the BBQ with the rotation it has taken the hassle out of my life. Rotating the bottles out with Home Depot I don't have to worry about out of date bottles needing re-certification. I can swap bottles on my schedule and not by urgent need.