I looked up the 28 footer and is claims a max of just under 13k. So I'd bet a pin weight of 2700.
Because I already had my 2013 RAM 2500 Tradesman, and under 550 weight in the truck, I was never over my GAWR, and never over gross combined. I was within 500 of max on the rear axle. I'd use the payload as a guide, but as long as I'm not over the axle ratings, I'd roll with it.
Load up with anticipated loads and got to a scale for official weight. I'd weigh my truck separately first.
With my learning, and knowing we will buy another 5th wheel, I bought a 2020 RAM 3500 Diesel Tradesman dually that will pull our anticipated purchase and be under 70% easily. And definitely under 80%.
2020 Ram 3500 Tradesman HO Diesel Aisen CC LWB Dually
2008 Cherokee WolfPack 295WP