Take the truck, full of fuel and every passenger you would normally haul, and without the tail gate, to a truck scale and weigh it. That is the ONLY way to truly know how much payload capacity you have.
Get separate weights for the front and rear axle. Most, if not all, of the TC weight will go on the rear axle. Subtract your scale rear axle weight from the door sticker's rear axle weight and that's how much capacity you have left.
You will probably find that your truck isn't capable of hauling a TC with as much room and as many amenities as you would like. None of them are. I would estimate that 80 to 90% of truck's hauling TCs are over their GVWR. My own rig tips the scales at 13,140 on a dually rated for 11,400.
2005 Chevy CC Dually D/A, 2003 Fleetwood 11X truck camper, 1997 Larson 206SEI boat