Isn't the OP talking about a TT so the only "payload" would be the tongue weight? as far as towing capacity/weight, that would be wet weight plus "stuff" not necessarily maximum trailer weight rating.
bumpy
Yes, but we need to know the payload of the tow vehicle to determine if there is enough left to handle the tongue weight. For example, if the TV payload is 1300lbs and 600lbs of that is used up for passenger weights and some cargo, plus another 75lbs for the WD hitch, then there would only be 625lbs left in the TV payload for the tongue weight. In this example that would mean a loaded TT weight of 4800lbs if using the average 13% for tongue weight (4800 x 13% = 624lbs)