1. If you are launching and retrieving the boat with the camper on the truck, it will not matter if you have the gas or diesel engine weight difference for traction. The weight saved with a gasser will add to payload capacity.
2. If you plan to travel west into higher elevations, you will be much happier with the forced induction of the diesel. Otherwise, you will be fine with a gasser and high ratio axles.
3. I don't think you will find large redistribution of weight between the standard cab and the extended one. The extended cab will weigh more and take away from payload.
4. Only you know if your extra you are packing will actually total up to that weight. If you take your boat often, the heavy items can be stowed there to free up your truck GVW.
