That's a pretty tall order... With the exception of the mpg, a mini bus with a rear door sounds like it might be a good solution. I think I'd want my bike(s) inside with me without cramping my camping arrangement. That might be tough in a van. I certainly don't have room for one in my camper van and it is an extended body maxi-van.
A garage model class C might be another consideration. Older, low mileage ones in excellent condition can sometimes be found pretty cheap, and no conversion necessary, but ready to roll and enjoy.