SJX426 

Couple of good solutions! I like the SeaSucker. If the roof is clean and smooth enough to keep the suction, your are good to go. For long term solution the other would work well too. The top of the camper is likely fiberglass. Any item mounted to it should have some sizable base to distribute the load/stresses and minimize stress points or small high stress areas. Flex will happen if mounted away from edges or features, or another way to say it, mounting in the middle of a big flat area is to be avoided. Use large fender washers with the bolts and a good sealer around the holes on both sides. Having the front and back mounting connected with a stiff (structural geometry) piece of steel or aluminum would be good.

MPG will vary slightly if the bike does not stick up above the roof of the vehicle. It may not be measurable given the extra weight of the trailer. I don't like roof racks due to the whole driving into the garage risk. Don't know what other things might be targets on the road.