To the O.P.: Three things.
1.The gas vs. diesel discussion is one that can be solved by knowing approximately how many miles YOU plan on putting on your rig. If over 150K to 200K miles, then a diesel is at the break even point getting the nod. If say, 50K to 130K miles, then a big gasser would be more appropriate.
2. The best win/win situation for you would be to find a late model something that has BOTH the truck and camper already mated. The beauty here is there is no 'period of adjustment' so common when you purchase each part separately trying to get the tie downs right, the suspension perfect, and the center of gravity working for you.
3. One added part is, 'how much off the beaten track' traveling are you contemplating? If the answer is very little then a 2WD dually with traction device would be the ticket. This will save you a lot of cash, both in initial cost and upkeep down the road. Most of the very high mileage diesel pickups I've seen with 400K to 600K miles on them are 2WD.