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

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.