As others have said - TW is TW, has nothing to do with a WDH. It should be 10-15% of the trailer weight.
As for payload, I'm a little confused by the question, but the true payload of a vehicle is the GVWR-empty weight. Don't know why those two numbers don't add up for you, but they should. Perhaps the payload quoted is making accounts for driver, or more likely you are looking at curb weight in a brochure (which is not the same as what your vehicle really weighs) or something else is at odds with the real numbers. To know your true capacity, take your truck with full fluids to a scale and weight it. Subtract that number from your GVWR and you have the total weight of EVERYTHING you can load on the truck including people, pets, gear, bikes, tools, hitch, tongue weight, etc...