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.

My Arctic Fox 811 has a 9' floor. 7.5' feet of it fit on my 9' deck before low hanging items interfere.

I've seen rigs where a short bed camper was used on a long bed truck. The gap between the camper would allow for storage, or, in your case, the tool box with fuel tank. This will of course limit you to only short bed campers which stop at about 8' in length.
I hope this is at least a 3500 truck, better yet a dually. If you've got a 2500 crew cab diesel 4x4 with the fuel tank even a small pop-up camper will put you way over weight.

You may be able to use a short bed camper on your long bed to allow for the tank, but the weight of the extra fuel plus camper may be too much for the truck without doing some upgrades.

This is my short bed camper on a long flat bed: