1st you have tires on the rear that are likely able to carry 6,000 lbs. Check the tire sidewalls for capacity at max psi. For accurate numbers, weigh your trucks rear axle with the entire family and pets, full of fuel. Then add in the weight of a 5th wheel hitch and any thing you plan on hauling in the bed. Subtract this total from your tires capacity and this is the pin weight you can safely carry.
Example,  My 5er's dry pin weigh is listed as 1500 lbs. Ahh but that nice big basement, the closets, cabinets it's now up to over 2500 lbs. It would be hard to keep a 10,000 lb GVW 5er's pin weight under 2,000 lbs.