It depends on what you want to do with the numbers. The TT will have a substantial hitch and tongue weight as toyhaulers are front heavy when unloaded but that will be less than the fifth wheel pin weight. Most boats do not have 13 percent or more on the tongue; most TT's do. What it all boils down to is how much over payload you want to be; that will dictate how much over gross vehicle weight you want to be as well. If you look at the rear axle you probably have plenty of capacity and many do consider axle and tire loads.
Others will try to stay within the GVWR.
I would think the fifth wheel would work far better with the dually as it has a larger rear axle and four skins down for the pin weight. Fifth wheel hitches can easily weigh about 250 pounds; add to that the pin weight and it can be considerable. For planning purposes use 25 percent of the toy haulers gross for pin weight. It most likely will exceed your payload without any other stuff in the truck or people. Never use dry brochure pin weights. An 18,000 pound gross toy hauler will likely have a pin weight around 4000 pounds if not more. And if the hauler is lightly loaded the pin weight could be even more as toy weights tend to balance out the pin weights.
