There are basically 3 ways to re-fill Propane tanks.
IF the tanks are horizontal mounted tanks bolted to the vehicle, You can drive to a refill station, Many RV parks have them or google PROPANE {zip code} and look for a wholesale house, USUALLY this is the best price, though I can tell yo uthe one in Seneca, SC is very expesive,, in Seneca, try ACE hardware.
You also learn to look for the big tank as you drive.
In some areas, the folks who deliver, will send a truck to you and re-fill on site,,, One of the many reasons I am happy that my major breakdown happened here (I can not drive the motor home due to a blown engine) not where my other major breakdown did (Dang near blew MY engine last fall, mine, not the motor homes, but got to repair facility (Hospital) in time) is that the local propane supplier delivers to the RV
Finally if the tanks are clamped on. Remove to refill location (NOTE: Driving to the pump house is always an option, or towing in the case of a trailer)