I expect a hollowed out camper shell to weigh 500-750 lbs. You will have to be careful what you add back in to keep within the rear axle rating. You have to take in account the weight of the truck occupants and any gear and supplies you want with you, so your payload capacity will be used up quickly.
I would not choose any camper that extends more than 2' rear of your truck bed. Longer models may not have have the floor support to hang farther off the end and they will also start lifting the front end up on the truck resulting in poor handling.
Think about how you will secure the camper to the truck. Will you bolt the camper floor to the tuck bed with sandwiched plates? Will you use the internal tie downs inside the bed? Will you use frame mounted tie downs with spring loaded turnbuckles? Each of these methods has a price along with pro and cons.
