In addition to what jrobe said, I would test the RV parts of the unit
Have seller turn on fridge on propane today so you can confirm that's working. Take a fridge thermometer with you.
Test all propane appliances:
light stove top
have owner start the hot water heater on propane. make sure it kicks in.
have owner start the furnace. make sure there's heat coming out
when you get back, test faucets for hot and cold water
test the roof AC unit
when you drive make sure you get on a highway and see how it handles at highway speed. Engage cruise control to make sure that works. Test the vehicle heat and AC at the dash.