We have carried our 2000 inside the camper but forward on the floor against the front wall. Turn it sideways and stick foam or a folded towel between the front wall and the gen.
Never smells. (keep the vented cap closed)
We have also carried it in the truck cab behind the front seat.
I would hesitate to have it at the back of the camper. Run into something solid or worse, have a head-on collision and that 45# generator will become a missile. Yeah, I know- one in a million, but why risk it when 8' further ahead would be better.
But up on the roof?? IMHO (and others) NOT a good idea.