Hi,
The 100 - 150 pound hitch will be totally supported by the tow vehicle, so it comes out of the GVWR of the tow vehicle.
It also comes out of the total GCVWR.
So lets say that the curb weight of a truck is 5,500 pounds and it is a 2014 F-140 with a optional 7,800 GVWR. The GCVWR might be 17,000, so you can tow a 'maximum of 17,000 - 5,500 curb weight - a driver of 154 pounds = 11,346 pounds. You are required to have a hitch, so also subtract that 140 pounds and you end up with only 11,206 pounds total trailer weight capacity.
If you add say 1,600 pounds of passengers, firewood, generator, perhaps a 250 pound shell, you will increase the curb weight, and reduce your cargo rating, and while your GCVWR is the same, the higher the curb weight, the lower the difference between GCVWR and curb weight will be.
So if the curb weight with a cab shell, firewood, and passengers is increased to 7,000 pounds, the hitch weight is 800 pounds, then your truck will be exactly the 7,800 pound GVWR of the truck. You can still tow a 10,000 pound trailer, because you take the GVW of the truck from the 17,000 pound total to come up with the truck's towing ability!
Good luck,
Fred.
