Here is what a lot of folks towing RV's end up doing.
1) throw the "tow rating" out the door. Towing 12500 lbs of flat load on a flat trailer (I literally mean flat) is not the same as towing 12500 lbs of fifth wheel. Aerodynamics play a much higher role with the fiver/TT. Also 20-25% of the fifth wheels weight will be on the 'pin'.
2) your GVWR minus your UVW (will probably be different than the posted curb weight, flashlights glovebox stuff etc, all add up) will be your "official" payload. (my FIL had a 99 Supercab PSD 4x4 lariat that had a payload of 1900 lbs). With a fiver plus gear in the truck you will most likely be over your official GVWR.
3) most find that it's not what you can tow, it's what you can carry. (Again, payload)
4) there is nothing you can do to increase the factory payload capacity. No matter what you do, the official GVWR will not change. However, you can increase the capability of the truck if needed, by adding F350 springs and increased capacity tires, airbags, etc.
Hope this helps,
Jeremiah
