I'm not sure how close to the rear edge of a RTT would be, but if the camera is positioned too far forward you might get the roof itself in the images and/or have difficulty angling it down enough to be useful.I plan to run with a roof top tent mounted full time (my Gren will not be a daily driver) and am wondering whether mounting the rear view camera just under the back of the RTT to keep it out of the elements will work. The view would be higher than normal, but the camera can be tilted down. I'll have to play around with that option after I get the vehicle and RTT.
Certainly a good idea though!