The wife has put down the foot, huh?
I've seen it done. Not a huge fan (no pun intended). For one thing, the fan really sticks out in the cabin. It looks a little strange. For another, it is more wear and tear on the lid. And lastly, while some fans can be left open while traveling, a wall-mounted Fan-Tastic fan certainly couldn't.
Use EternaBond tape over a Proflex RV fillet sealant over a butyl tape compression gasket, and you won't worry about roof leaks.