A SRW pickup is good for about 4000 lbs payload on the rear axle (around 7000 lbs RAW) - Any heavier and you need 19.5" wheels or a DRW. This translates to a camper with a dry weight around 3000 lbs before you add gear and water.
To just carry a camper around, you do not need the additional cost of a diesel engine. It is nice to have the additional fuel tank range of a better mileage diesel and exhaust braking on steep downgrades, but it is not a requirement for your application. You really do not gain or lose payload with either engine because the empty rear axle weight does not change much - You will have people spouting off about GVWR, but the real concern is what your tires, rims and rear axle will carry.
