It also depends on what you final goal is. If all you want to do is haul a 5th wheel RV, you only need to do a few things to a class 8 truck. Add a Jackaloppe for the wiring converter. Add a brake controller like a Tuson Direct Link. Add fenders to protect the RV, either move the commercial hitch towards the rear of the truck so you can turn the trailer without hitting the truck or better yet replace the commercial big hitch with an RV style air ride hitch and mount it at the rear between the frame rails. This part may be the hardest as you typically have to add a little bit of frame to do it.
Anything else you do is just extra like making a hauler bed, carrying a smart car, adding extra water tanks, adding a generator, adding storage compartments, etc....