Terry, let me make sure I understand GVWR correctly. GVWR is maximum total weight of vehicle, cargo, people, fuel, and other fluids together. Correct? So if I take my truck to the scale, I'll get a weight of each axle. As long as those axle weights do not exceed their ratings and the combined weights don't exceed the GVWR, I'm ok.
If I weight the truck with the WDH disconnected, I'll see a very large increase in rear axle weight, and a decrease in front axle weight. With the WDH connected, a much lower weight will be added to the rear axle and weight will now be added to the front axle as well. The combined axle weights with the WDH connected will be less that with the hitch disconnected. That difference is what gets distributed to the trailer. Why should that weight still be factored into the GVW calculation? It is not on the truck anymore.