Problem is, putting together the information is a lot of work, and then you have to start putting qualifications into the list: always attended, can fit what size vehicle, towing what else, can I get to it without backing up, etc.
There are 2500-3500 filling stations set up to sell LPG as a motor fuel, and there are associations that work to maintain those lists. There are probably 5,000-10,000 full service LPG dealers, but not all of those operate retail filling stations, though many could handle a RV at one of their storage sites and might take care of you by appointment. But most stay busy carrying LPG to their permanent customers, many of which are retail operators.
Ah, retail operators: U-Hauls (and other rental operators), RV dealerships, hardware stores, gas stations (many in the ""filling station" lists), farm and ranch supply, feed stores, RV parks, places with a small tank and a metered pump. Possibly another 10,000 or more, some can reach a vehicle but may not be big rig pull through, some only fill bottles, though that business is quickly being smothered by the ubiquitous exchange bottle outlets.
When traveling, I tend to refill at campgrounds, when LPG is available and before the need is urgent, though price may not be the same as a wholesaler, it doesn't bother them to make the sale if I'm staying there and on the way in or out. At dealers and distributors, often have to get a delivery person called to the office to pump, and in many cases the location is either back in or back out.