For offroading and handling a light load like a 3,000 lb. camper there is not benefit to a diesel engine. The extra power you do not need and offroad I worry about both range per tankful of fuel and how easy it is to find a gas station that is open. With diesel found only at about 20% of gas stations it definitely affects route planning.
The extra cost of diesel is $9,000 for the engine and Allison transmission. It also adds about 800 lbs. of additional weight and offroad lighter is better. I would go with the GM 2500/3500 as with the 2011 and later model years the frame is the strongest and stiffest you can get. The independent front suspension also makes the ride tolerable and is much better than a solid front axle on rough surfaces.
GM has a tow package that is useful even if not towing. The tow haul mode is great with a 3,000-4,000 lb. load in the truck (passengers, gear, water, and camper).
With any camper I would take the published weight and add 1500 lbs. to find the worst case load that the truck will be carrying. With GM trucks the 1-ton's can handle a payload up to 6,000 lbs. depending upon the configuration. The payload capacity from the manufacturer decreases based on the expected passenger load. A crew cab to that holds a driver and 5 passengers will show a reduction in the payload of 4 x 150 lbs. or 600 lbs. compared to a regular cab pickup.
For offroad I like as short a wheelbase as possible and went with the short bed truck with an extended cab. At the other extreme is a long bed and crew cab truck where turning radius is almost an oxymoron.
The Z71 "offroad" truck models often have 20" rims and tires and a greatly reduced payload capacity. Get the 18" forged aluminum wheels and you will have a greater payload with the factory tires and more options when you decide to replace them.
