IMO...4 gallons is just enough to line the bottom of the tank. Fill it to 1/4 at least. The fact that it runs on LP eliminates the ignition system and isolates it to a fuel delivery problem. If its been run on LP the entire time with previous owner, you may have bad seals in the fuel system, pump in tank is seized up, filters clogged or a host of other issues. What type manual switching system do you use that changes back and forth from LP to gasoline? Look for model numbers or some type of ID on that system and contact the maker of those parts...
2018 THOR Chateau 35SF
Two Labs, Bugsie & Blondie