Since a truck camper puts nearly all its weight on the rear axle of the truck, you can work with a couple of educated guesses...
Generally speaking a 2500 class pickup weighs in around 2800-2900lbs on the rear axle empty with nothing in the bed. The RAWR is around 6100lbs. That leaves you about 3100-3200lbs of payload before you reach the axle rating.
Generally speaking a 3500 class SRW pickup weighs in around 3000-3100lbs on the rear axle empty with nothing in the bed. The RAWR is between 7100 and 7400lbs depending on the year of the truck and how it's equipped. That leaves you about 4000lbs of payload before you reach the axle rating.
Generally speaking a 3500 class DRW pickup weighs in around 3200-3300lbs on the rear axle empty with nothing in the bed. The RAWR on DRW trucks can vary quite a bit between 9000 and 10000lbs or more. But, you get the idea here right?
Basically you just need to know the RAWR, which you can get off the door sticker, to make an educated guess on the truck's payload.
That said, you will probably want to add some suspension helpers when approaching these heavy weights, because the truck is not designed to sit level with a full load, and a level truck drives better.
