If your not running stock tires, then why use that number for "never exceed". If the tires are capable of the load, then the GVWR is your number to not exceed. But then, you have never exceeded a speed limit either, right?
It's not the tires that are the limiting factor - they almost NEVER are the limiting factor when it comes to weight in a 1500, or any half ton truck.
It is ALMOST ALWAYS the springs/frame/axles.
You've got a RAWR of 4100 lbs, and it's NOT your tires that are dictating this. It's the weight of the truck (including all options) versus the axle/springs/frame. I don't know how many ways we can put it out there.
I've been a part of this discussion here and on other forums, and the consensus among those who look at and understand the numbers is that there are VERY FEW half-ton trucks that can pull a VERY FEW number of fifth wheels and not be over payload/axle weight ratings depending on which number is more important to you. Most of those half-ton trucks are unicorns with some kind of max payload package and very few, if any, options. Most of those fifth wheels are less than 28 ft in length and MIGHT have one slide. Pretty much any other combination and you're outside what the truck was designed to do, and you're not running safe.
For other people's sake, I hope you're never in an accident, because there's a good chance you're going to ruin their life. You don't have a truck that can handle the extra weight of a fifth wheel but insist that you can do it. And sure you can, until you can't.