In my experience with my family of 4, 1500 lbs is a reasonable estimate. But the big thing is how much water will you be taking? You can easily gain or shed 400 lbs depending on how much water you carry.
Also remember this, things like that 100 lbs toolbox when carried in the bed of the truck add 100 lbs to the payload. Carried in the TT over the axle and it only adds 12 lbs to the payload. Point being, load that TT up and carry less in the truck.
Go to this site
HERE look on the left side of the sheet for a button that says "towing capacities", it will give you a handy xls worksheet for figuring this all out.