I'm not sure what an "all bed" camper is. The manufacturer will state if the camper is designed for a short bed (6.5') or a long bed (8'). A very few models are designed to fit either one. If the manufacture state a camper fits a 6.5' bed and that's what your truck has then it will fit fine as far as the bed. As you note, you likely need a 3500 series truck for most any hard side truck camper.
My last TC was an 11' model that I carried on an 8' bed. It was actually a bit longer than 11' as many of them are. I had a 48" hitch extension to pull the boat and the ball was barely behind the camper bumper.
Any truck will "sag" when loaded because the springs compress. Just the spring compression isn't necessarily bad. It depends on how much weight you're hauling versus how much the truck's tires, springs, axles, etc. can handle.
As for fit, Ford cabs are often too high for older campers and the camper needs to be lifted on foam insulation or plywood to clear the cab. In 2007.5 GM narrowed their tailgate openings a little so older campers are tricky to load because of that. If you have a small, narrow camper that is only 7' wide and want to put it on a dually it will be difficult, even with dually brackets, to get the jacks far enough apart.
