Slide-in Truck Camper

I am building a composite pop-up truck camper that will slide into the bed of my truck. I can give a full list of structural materials that I will be using and have a basic SketchUp of the design. It will be made from structural foam with vacuum infused fiberglass on the exterior portion. The two issues I need help with is figuring out where the center of the weight will be located within the truck bed and if any additional support will be needed within the overhang portion that we will sleep on. It will have a 72" overhang over the cab of the truck and will need to hold at least 500lbs plus movement. The current design is to only have the structural foam and fiberglass underneath the sleeping space. I am trying to keep the camper under 1200lbs so it will be easier to travel with. I have a few pictures attached showing the basic design.