Instead of the in-bed fuel tank you should consider one of the larger tanks that replace the stock unit from a company such as TransferFlow.
Almost every camper
Outfitter builds is semi-custom and hardly any two are alike. They can accommodate pretty much anything that doesn't compromise the structural integrity of the shell but such work doesn't come cheap.
They have built two for me.
My 2007 F350 SRW/4WD/Diesel carries my fully loaded Outfitter Apex 9.5 and still sits slightly tail high (with no suspension mods) and while I have no idea what my buddy's 22ish foot TT weighs my truck towed it just fine at the same time as I was carrying my camper. Truck sat right at level without the use of a weight-distribution hitch. I believe I was at my GVWR but under my GCWR.