Go to this linkpick your new truck.
click on base weights/GVW Payload/Trailer tow
on 2500's the highest payload is 3990 on a 4x2 reg cab long box gas engine truck.(and that 3990 has to include you and other passengers plus gear in truck. Highest 2500 GAWR rear is 6500
now on 3500's, payload is still around 3930 on Single Rear wheel trucks with rear GAWR of 7000 lbs.
move up to dually and your payload and rear axle weight jumps around 2500 lbs or more.
click on Camper Guide
This will show you "remaining cargo" which is after the ram engineers deduct weight for what a max load of passengers would weight. Most of the 2500's are around 1500 lbs.
even in the 3500's to see numbers of 4000 lbs your looking at GVWR of 14,000 or 13,300 which I have a feeling are both duallies.