Hmmm...
Filling the onboard tank on a motorhome...
Here in Billings, the first source that comes to mind is Amerigas, the propane distributor.
Now, let's see, places that have fairly easy access for a 32 foot motorhome.
Closest to me is Big R Ranch Supply. I fill the coach there often.
Tractor Supply has propane.
Ace Hardware has propane.
U-Haul has it.
Flying J has it. I'm not sure, but I suspect that the Pilot on the other side of the freeway has it.
Bretz RV has it.
I think Pierce RV has it.
Those are the ones locally that I can think of right away. I'm sure there are others.
The point is, we have never found filling propane to be a problem, either with the 20 pound cylinders or the frame mounted tank on the coach. The facilities are all over the place.