Propane filling

I would try Google Maps and search on "propane refill"

This doesn't do to bad. If a campground doesn't fill tanks then ask them. Haul, tractor supply and others often can fill them too.

I use an Apple app by Allstays, called Camp and RV. Use it for campgrounds as well as finding propane, etc. under the filter category, propane is one of the many listed.

I do not know of any such app. Around our neck of the woods, just about all the gas stations have the ability to re-fill your tank, bottle, cylinder, or whatever it's called. I have used the old outdated yellow pages on occasion.

This might work:
https://play.google.com/store/apps/details?id=com.omecha.propane&hl=en

I wish.