A diesel and any sort of 'extra' cab space reduces payload.
Diesel are good at proving torque. Torque is needed when pulling a load. You will be carrying a load not pulling a load.
Payload is the critical item.
You should buy a camper before the truck then you will know the payload the truck can carry before you buy the truck. If you are set on a 'big' camper you will need a 350 dually.
I have a 250 single and it carries my 10' rig nicely. With airbags and upgraded rims and tires.
My advice wold be to buy a gasoline standard cab.