Your fifth wheel GVWR has no relation to your truck GVWR. Your 12K 5er weight does not leave you 3K lbs. for passengers and pin and whatever else you put in the truck.
For the F250, the GVWR is either 9800 lbs or 10K lbs. Assume you are looking at the 10K lbs. version. With a diesel engine, and the FX4 4X4 you mentioned in another thread, and the supercab or crew cab (you mentioned carrying more passengers) your truck will weigh at least 7400 lbs. Probably closer to 7600 lbs. I have a similarly equipped truck and mine weighs over 8K lbs.
This leaves you with a MAX of 2600 lbs for payload (10,000 - 7400 = 2600: GVWR - actual weight = payload).
Your 5er will have a pin weight of between 2K and 2400 lbs given the specs above. That leaves you with between 600 and 200 lbs. for passengers and other stuff in the truck, including the hitch.