"Where does one get a propane fill in the USA?"
Many RV Parks, gas stations, farm supply, home and garden supply stores have it. Watch for signs on the freeway. Next time you find a source, they usually have a blue sign with specific lettering. If you train your eyes to look for the vertical and/or horizontal white tanks, it gets easier to find. The tanks are usually outside and at least about 50' away from the main structure they are associated with. It's a good idea to use google maps to check access since you'll probably need generous space to get your rig near the tank. Be careful, filling your propane tank(s) is a really good opportunity to damage your rig, the adjacent fill area, or both.
Chum lee