Going out on a limb here... drop spare tire, add genset?

Sounds feasible, although the devil is in the details.

My first thought was spare tire to the roof, but that would be much more difficult than the front hitch suggestion above.

You would probably need to fab up some splash guards and things for the genset to keep road debris off of it. Also might have to keep ventilation and cooling in mind.

The fuel tap shouldn't be that difficult. Should be lots of shops around that do things like welders in the truck bed who would be familiar with that sort of thing.