common sense. Yes, you can leave on propane. If you have 'auto' just leave it on that. I suspect your refirg is OLD.
as for level, again, use common sense, you don't have to spend hours making it perfect. if you can sleep/walk in it comfortably than it should be good to go.
BUT, if you have a 3 way (one of them 12volt) just don't use unless you have lots of batteries.
I didn't even know they made 3 way refrigs anymore.