Artum has the size about right.
Take your truck to a scale loaded with fuel and passengers and weigh the rear axle. Subtract that weight from the RAWR on your door sticker. This is how much additional weight your rear axle can carry. Most people figure about 1,000 pounds less than that for the camper's dry weight. This will allow room for you to add water, propane, food, dishes, clothes, etc.
I'll warn you now, you'll probably be disappointed with the size campers this puts you in. If you were thinking of 11' campers with multiple slides they're too heavy.