1981 Cadillac V8 (with computer to shut down 4 cylinders when cruising) - HOLD MY BEER....Ford 6.0. Definitely. The shittest engine ever produced.
We had a 6.0 PowerStroke in our Excursion, and got 18 MPG not towing (never towed anything). We could get 20+ MPG with a good tailwind, but strong headwind knocked it down to 16 MPG. I did have a mileage tune in it though. And, I soon as I bought it (used), I put in a BulletProof EGR cooler. I had it for 60,000 miles.
For the OP, I'm not much into campers so I don't know how big yours is, but I drive interstate alot, and I see alot of 3/4-ton pickups struggling with 5-th wheel campers, particularly in windy conditions. It is the tail-agging-the-dog syndrome. I'd consider a 1-ton if you are going to be extensivley pulling this camper.
This is why we went to a dually in 2016. WAY more stable pulling the camper. Upgraded to the F450 this year with the wider front axle for a tighter turning radius. It is a fantastic truck. Our 5'er is 16K lbs dry and has a 3300lb dry pin weight though.