If you ONLY use your pickup to haul your camper, then unless you tow a heavy trailer I can't figure out why you don't choose a Class A or Class C motorhome.
The 2WD/4WD question isn't about whether you go 4-wheeling or live in Antarctica. That situation is a no-brainer.
The low-range gears are very useful (especially with manual front hubs that let me effectively have 2-Low) when loaded heavy on an incline.
The ability to have the front climb up a leveling block instead of pushing it is nice.
If you ever drive your truck without the camper on it and ever pull a trailer, try to get moving from a stop if you're on gravel, or maybe even a wet street.
I've had 2WD pickups and lived to tell about it but unless it was gonna be a lowered, tricked-out cruiser, I'd never buy another. It has NOTHING to do with going off-road. There are just too many times a 2WD pickup sucks ON the road!
I won't even beat the dead horse about driving in snowy or icy conditions.