Payload is only part of the truth, and not the most important one w.r.t. truck campers (and there are a lot of discussions about weight on this very forum).
You need to keep an eye on the Rear Axle Weight Rating, as most if not all of the camper weight goes to the rear axle, especially in a short bed truck. Ideally, you should get the actual axle weights of the empty truck, as in, weighed on a scale. The difference between empty rear axle weight and RAWR tells you what you should be able to haul.
The limiting factor of the weight rating is often the load rating of the tires on an SRW truck.
Be very careful with any weights that come from a manufacturer's spec. sheet, be it truck or camper. They are often very optimistic and do not take all weights into account - you need to know what is in and what is not (truck trim line, camper jacks, A/C for example).
