[wingnut60]

landdawg,
Your tires are about as good as it gets in a 16" tire for load capacity--there might be some higher if you went to a larger 16" but not much to gain.
So, you have 7500lbs of tire capacity on the rear axle--this should be the maximum weight on the axle at any time when you include all inside and bed contents AND the pin weight from any fifth wheel you want to use. But it needs to be said--if you run a tire near or at its load capacity, it has to be at the psi shown on the sidewall; even then, the tire life will be lessened if run at max load all the time. This is just like anything else--if you stress an item too long at max load, it will probably fail sooner than later. Don't mean to scare you too much, but the internet RV forums are full of examples of blown tires on trailers and the resulting damage.
Good luck in the search for a fiver that will fit the truck.