crewcab longbed tool/fuel box in bed looking for camper idea

Axle location from the rear of the truck bed is the same for both short bed and long bed trucks. The different 18" in bed length is all between the cab and the axle.

Therefore, if the fuel/tool box is about 18" wide, a short bed truck camper will work perfectly. However, make sure you have the cargo capacity to carry the weight of the fuel and camper. Weight the truck with all tanks full of fuel, normal gear in the tool box. Use that along with your GVWR and GAWRs to determine what you can carry.