It is a hard decisions, but lets see what the numbers say. Base PW payload is around 1450lbs, with added options it is even less. If you step up to the similar trim 2500 Diesel you are gaining only about 800lbs in payload (about 2200lbs). Now since you mentioned you would add all those off-road accessories that PW has (winch, lockers, winch bumper) then your gain in payload disappears very fast. So in reality after upgrade to 2500 Diesel you just spend all that money without not much gaining in rated payload. This is mostly due to additional weight of the Diesel engine.
If you replace your truck with standard 6.4l 2500, your payload difference would about double. (about 3000lbs) Again, add some of the off-road accessories that cause reduction in payload and you are still have fairly significant payload gain. Next step up would be upgrade to 3500 SRW, but those still have old suspension in the rear if I'm correct.
The reason for significant reduction in PW payload is suspension. The coils in the rear are softer so the truck performs better off-road. The frame, axles and other items are the same as rest of the 2500 family. Not sure if they still do, but PW usually have a warning insert in the documentation that slide in camper is a NO-NO. This is due to soft suspension and possibility of tip over while off-roading.
If it was me, I would keep the PW and add airbags as you are planning. This would give me stock suspension while the truck is not with the camper and added payload while the camper is on.
In summary, if you want better fuel economy, go Diesel, but keep in mind you will not gain much payload. If you want payload then standard 6.4l 2500 is your best bet, but trading PW for standard 6.4l 2500 is a nonsens.
Keep the PW, add the bags and call it a day. Even with other two versions many would add airbags to the rear suspension.
One other thing to keep in mind is vehicles factory GVWR rating. If you overload the truck (even if it can handle it no problem) and God forbid something happens on the road you might have to face some legal consequences. It all depends how comfortable you are with this fact.