Put the extra fuel tank in the TT.
Toy haulers commonly have fuel tanks for the toys, so it should be easy to get one installed in the basement of your custom TT.
This way gets rid of several compromises in the truck. You can have a larger camper that sits properly in the bed and is tied down properly. There will be less weight on the truck, which makes the SRW more suitable with fewer suspension add-ons.
Maybe you won't even need 19.5" tires. You may still WANT them for their durability, but you won't need them. They become extra cushion and peace of mind, rather than an absolute must to keep the truck right side up.