But, most 5th wheels have 20-25% PIN WEIGHT, so with only #1270 payload before there is any actual body in the truck, you'll have to settle for a pretty small 5th wheel if your goal is to not exceed your trucks weight ratings...
Once I have my truck loaded up with stuff in the bed and my #230 ass in the truck, plus the wifes, which is an undisclosed amount

I only have about #400 of rear GAWR left and about #600 of GVWR on my truck... The tongue weight on my #5000 fully loaded trailer is around #650.. So, I'm FAR from the trucks "tow rating", yet I'm not far from exceeding it.. I figure I could go to around a #7500 fully loaded trailer and be right around the max ratings of my truck..
I have to pay for #8000 GVWR tags on my truck to register it in WA state.. So, as far as my trucks GVWR, I can go to #8000 because that's what I'm actually registered for..

But, I'd exceed my rear axle ratings long before that, since I'm already only #400 under it as it is..
Anyway, good luck with it.. You are just going to have to decide what weight ratings you are going to go with and you will need to get some actual weights to determine that, since noting weighs what the brochures say they weigh..
Mitch