You'll need to drive your rv next to one of the big tanks at a rv store, hardware store, or anywhere that refills propane tanks. I'd call before driving over there to make sure you'll be able to park next to their tank.
If it is hard mounted to the frame and likely mounted in a horizontal configuration, it is a liquid tank designed to feed vapor to the appliances. If this is the case, there will be a liquid fill fitting on the tank with a 80% fill spit valve. The liquid fill fitting will be accessible from the outside of the RV, likely behind a small door.
This type of tank is filled from a bulk type propane filling station. If the service station can only fill by weight, you need to find another station. A lot of gas stations have a propane filling station that can fill by volume. It is still in use today.