Look this is really really simple. Load the truck like your going camping. Drive across a set of scales. Get your true actual front, rear alxes and your loaded weight.
Subtract your GVWR, FAWR, and RAWR and see what yoy have left over.
Right off the bat we know your tires are only good for less than 6300 pounds.
Depending on how you load, your scaled truck weights are going to be around 3500 rear axle and 7500 pounds total weight. But until you get real weights it is all a guessing game. Dont forget to add 250 pounds for a slider hitch to your scaled weights. Typically a fifth wheel will have around 20% but can go as high as 25% of its loaded weight. Rarely will you find them less than 20%.