An older F250 with a diesel will likely not have much payload capacity. I'm guessing 2,500 pounds or less.
I would take the truck, fully loaded with fuel and passengers and minus the tailgate to a scale; get front and rear axle weights. Subtract your unloaded rear axle weigh from the RAWR on the door sticker. This is the most a camper and it's contents should weigh.
It's possible, particularly with Fords, that the RAWR is largely determined by your tires. You might be able to upgrade the tires and gain some capacity.
Either way I think 8' campers or possibly light 9' campers would be about the limit. If your looking for used campers a Lance 835 or an Arctic Fox 811 might be okay.