OP, once you get all this payload stuff worked out to your satisfaction, you're going to go through the same thing trying to determine the true empty/loaded weight of any future camper.
Many recommend adding 1,000 lbs. to the "empty" weight of your camper to arrive at an approx. loaded weight. That's reasonable. However, good luck finding out the true empty weight prior to purchase. I've witnessed a few campers visit a CAT scale over the last year or two. Got to say I was mortified with some of the numbers. Not at all what I expected based on the manufacturer's empty weight claims---even those manufacturers that supposedly weigh their campers individually off the assembly line are suspect.
A lot of smoking mirrors out there. Good luck.