I've not found an app or website that lists all LPG retailers let alone distinguishes them as to whether a large RV can get to the pump as a pull through, or do so without backing up. Most lists are "there is a dealer at this location" which may or may not work fir you.
A big issue whether the retailer is conveniently open, as many keep their service people on the road, and you may need to call for an appointment to meet at the pump. Probably 2/3 of the LPG bulk dealers in my part of the country are unmanned at storage and pumping locations, and smaller 2nd or 3rd tier retailers often don't have room around their pumps, or need a RV to back in or back out.
The most convenient locations I've found. have been at RV parks. Because they service RVs they are set up to be convenient for drive through, and are usually happy to sell you LPG if you are on the way in to stay for the night.
Tom Test
Itasca Spirit 29B