Heres a simple and very useful addition I made to the camper shell on my truck and it get used all the time.
The concept was to add USB ports and a place for future switches to the rear of the truck.
With this simple addition I now have two USB ports for charing devices as well as a way to monitor my starter battery while running the camp lights. I plan to add additional controls for a new camp light and interior and exterior temperature gauge to further up the level of comfort in the sleeping setup.
I started by making a quick mock up
Using cardboard i cutout a quick mock up to fit into the area i planed to place the the panel. I often find that using this method is more accurate then measuring and offers your insight that you can only get when
placing a 3D Item into your previously 2D plans.