Do not know of any toy hauler 5th wheels for that truck, there was a post regarding a grey wolf 25rr that would be close .
The airlift kit will not increase your payload, it will take some of the squat out of your truck. Your payload is the number on the sticker and nothing aftermarket will increase or change that number, some people are bothered by this, state in an accident you could be liable to fault for overloading a vehical. Others ignor and do not care.
I towed about 8000 pds with a 1500 ram, airlift kit and erased tires. Towed ok, not great, found it to hard to travel long distance without getting tired.