Welcome to the forum.
I was (and may still be if the Nissan is not what I expect it to be) in the market for a Ram 1500 diesel, so I have done my research on them. First, you can't increase payloads or GVW. There is a big cushion between axle ratings and GVW on the rams, though.
2nd,you got a shorter than short bed with a rambox. A convebtional fifth wheel will not work because of the narrower beds. Guys with 2500 ramboxes have used reese revolution pinboxes with success.
My opinion, look for a conventional ball trailer that the 1500 will handle or get more truck.