Hootie - I mounted the camera on the back wall above the middle light (not on the roof). When I drilled the 3/8" hole (I think that was the diameter) I found part of the wood stringer that runs along the top back of the unit. So the top 2 screws are anchored in the wood and the bottom two are self tapped into the wall/insulating foam. Put silicone in all holes and on back of mounting plate and it turned out pretty well.
All that said, if I'm able to return the camera to Camping World I plan on taking it off and going with a wired system. The Voyager wireless is flaking out way too much when driving down the road. While it works OK for backing up into spots (actually, the camera is a little more fish-eye lens for my liking as well) it's unreliable as rear view camera while traveling. As the signal weakens the frame rate drops and gets pretty choppy - then if you lose signal altogether the screen either freezes or blanks out. Not what you want to see happening if you're trying to get a sense of the vehicle in the next lane.
BTW - for reference, we have a Momentum 355 with a GMC 3500 crew cab dualie and 8' bed. We might be just on the edge of the transmission range.
My recommendation to anyone checking out this thread - while it will definitely be more work, if you can do it - go with a wired system. As always, YMMV.
Best of luck!