Packing our last RV for a family of 5 added about 1,000 pounds and that was a little 11' truck camper. The dry weight of the trailer won't include batteries, propane, water in the fresh tank, water in the water heater, water in the waste tanks, and possible some dealer added options.
Weigh your truck axles with it full of fuel with the family on board. Subtract the rear axles weight from the RAWR and that's how much you have left for a FW hitch and the pin weight. Figure the pin weight is 20% of the trailer's loaded weight and you know how much you can tow. This number will likely be much lower than the truck's advertised tow rating.