I have not read all 8 pages, but an important number is on the tire loading door sticker. It should read do not exceed xxxx lbs, which includes all weight in the truck, and any hitch/tongue wt also.
If you subtract that number from the GVWR 7,200, it is the wt of the Burb when it left the factory. The not to exceed number is your available payload.
If you loaded the truck with everyone/everything in it to go camping, and weighed it on a scale, you would know what was left for hitch and trailer tongue wt by subtracting the scaled wt from the 7,200 GVWR.
Hope this helps understand payload.
Jerry