A 11,000 GVWR 5th wheel will have a pin weight of 2000# when loaded for camping
Your payload of 1285# would be closer to 1000# after installation of 5th wheel hitch.
1000# of payload doesn't leave much in the way of 5th wheels due to pin weights.
But your truck is quite capable of towing a very nice travel trailer.
I applaud you on going and weighing truck so that you know how much capacity you have for weight.
I had a 2001 F150 7700 GVWR.
Ran out of payload quickly.......19' 5th wheel
BUT it towed my big heavy boat just fine