No, thats not exactly how it works. You find the cargo/payload (what you can fit in the bed, in the cab, and tongue weight of the hitch all together) capacity by getting the trucks GVWR and subtracting the actual weight of the truck, and all the people, fuel, cargo etc you will be carrying.
This will change depending on what year, cab configuration, 2500 light duty, 2500 heavy duty, etc. Give more details about the truck and we can help you better.
For example both of my trucks are 9,200 GVWR, but the extended cab gas truck would have a higher payload capacity than the crew cab long bed diesel since it weighs less and takes up less of that 9,200 GVWR, but either way I would have over 3,500lbs payload capacity in either truck, but subtract a couple hundred pounds for a full tank of fuel, lets say 4 people in the truck at 150lbs each for another 600lbs, 200lbs of cargo, another 100lbs of fresh water in in the camper, etc, and we are already getting down to only 2400 lbs capacity left for the camper...get it?
Now then, a light duty 2500 and many OBS 2500s are usually around 8600 gvwr, so right there you already have around 600lbs less capacity to work with depending on curb weight of the truck.