I hate all the hats and mushrooms on a typical camper, or rv. I would go for a side wall, but
hope it is the side away from the door and not near any windows, if you go high up on the wall I would think it would be OK. Heck you could even fashion a snorkel to raise it over the roof if needed. Yea membrane issue would be the main reason I would go out the side wall. If it were a metal or fiberglass sheet roof then roof would be the choice for the exhaust which I imagine is small compared to the huge bumps on the typical vents that we all put on our rigs.
RUSTIC is good.
Kudos to those who make Local, State & Federal Parks & Campgrounds possible and to those picking up the slack by Providing Private Campgrounds.