I agree that you are trying to over figure.
Your GVWR and Tire ratings are important for safe operation. GVWR ASSUMES everything in/on that vehicle including a toothpick.
What you need to make sure is that whatever tires you have on that rig are adequate for the TOTAL WEIGHT LOADED. Don't assume anything. Get down on your hands and knees and READ the weight rating on those tires and use them.
You should notice that when using single rear tires, the weight rating on that tire is different from the weight rating for duels on the same tire.
I agree with the poster who said to take the rig fully loaded (I don't mean what you THINK you may haul) to CAT scales and weigh. Oh yes, that includes passengers, dogs, cats, etc. Not only weigh each axle, but each tire seperately, and an overall weight. Good rule is keep the weight under 80% of the GVWR.