I just finished my rear camera installation.
Don't drill until you are sure what is behind the gelcoat and fiberglass. Don't ask why I am stating this.
I found that in my rig the top roof cap has a void behind it where the roof lights are installed. I removed the center light and found a nice large hole already there where the light wiring came through the roof and was later stuffed with caulk. I just cut a hole in the caulk, ran the camera wire into it, cut a small hole in the wall fabric (inside a rear cabinet) and then re-caulked the hole. The camera cable fit under the light housing but I did have to carve our a notch in the housing for it to fit flush on the roof. Once done, the light housing was re-caulked.
On the inside of the coach, the cable was run from the rear cabinet behind the wall fabric over to the rear passenger side (PS) corner of the coach where I ran it down to the floor and then along the wall inside and exiting hollowed out molding strip..over to the shower..under it to the furnace..through a hole in the floor and up the PS rail to the PS door where an existing wiring compartment existed. I just drilled a new hole and ran the cable inside ther compartment..into the kickpanel..up the door pilar and into the headliner.
Be sure to put the cable in loom and secure it at least every 10 inches or so to protect it.
Works great!