LP is difficult and there is no one good app that I have found. In addition to FJ/Pilot stations (use their app to make sure that station sells it). I use the following three websites to plan ahead for LP.
Amerigas.com
Surburbanpropane.com
uhaul.com
Most larger commercial campgrounds also have LP and it usually is not that much more expensive to warrant searching.
The biggest problem I have found with LP is finding someone who knows how to fill an RV. One time out of three it seems that the individual does not know what they are doing. Last summer, before I knew it, one dummy had filled my tank to 100%. It took 30 minutes to bleed it down to 80%. Watch them like a hawk.
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