Honest Don, if it was that easy that would be nice. You are going to have to weigh your truck with everything you put in it, kids, pets, tools, fully loaded with fuel, then see how much real axle weight you are at, and that will determine how much more you can put onto the real axle.
There truly is good reasons why 3/4 ton trucks are great towing machines for trailers, but the pin weight of a big fifth wheel can eat up that axle capacity fast.
First step, get it weighed on a good scale. Really see what you have left.
Your real axle capacity is not the available left from your GVW, it's your tire carrying capacity. Look at the max weight on the rear tires. Fifth wheels carry 100 percent of their pin weight on the back tires. Campers do not. Trailers with equalizers do not either.
Edit again... if your tires are stock, they are rated at 2337 pounds each for 245/70 r 17. Double that to find your real axle capacity.