If you want what the factory says is "OK" open the glove box and find the piece of paper with the factory load rating, this gives you what the truck can carry with all occupant seats filled with 150lb people and full fuel etc. If you bought used hopefully it's still there otherwise see next paragraph.
If you want to be ballpark add the front and rear actual weights and subtract from the GVWR, this gives you what all cargo can be. If you don't have all seating positions filled you will have more cargo allowance. Also figure you'll have 500lbs (minimum) added to any advertised weight of a camper for gear. Also some campers are weighed "dry" which means no propane or water, Lance weighs with full fluids but no options.
No truck has a GVWR equal to the front and rear axle ratings added up. Mostly due to the fact that it would be almost impossible to load a truck so precisely that both axles were loaded to capacity and neither was over.
Many folks are over the GVWR but under the axle ratings, most of those have done upgrades. If you want to be completely "OK" then stick to the manufacturer's ratings.
Good luck on your hunt, TCs are a great way to get away.
Dave
(900lbs over GVWR but under axle ratings, many upgrades)