2015 Ram 2500 or 3500 6,7L, SB, GAWR question

Your stated GAWR for the RAM is 6200. You need to know the payload rating. That 6200 number does not allow for the weight of the truck on the axle which probably leaves the payload at 3000 or so depending on specific truck and options. My 2012 F350 SRW payload rating was 3450 lbs, which was too low for our 14,000 GVWR Bighorn which has a loaded pin weight of 3400 lbs. by the time I added other cargo and passengers, (plus I wanted an auxiliary fuel tank) I was well over the 3450 posted limit. I now have the F350 DRW with 5400 payload rating, so I was able to add that fuel tank. A truck for Fifth wheels is all about payload rating. If the truck is rated to handle the payload, all the other numbers will fall in line. A 2500 does not have the payload rating you need ; A 3500 SRW will be borderline depending on how you load.