Allow me to muddy the waters a bit further
After my Excursion blew up, I traded it and bought a brand new Dodge Ram 2500 4-door long bed 4x4 with the Cummins turbo diesel.
My dad's best friend has owned a Cummins powered Ram since they first came out in '89. He routinely puts 300,000 miles on them and trades them off, still running great, on a new one about every 8-10 years.
My dad bought his '99 new in '99 and has had no problems of any kind with it.
OK, so I buy mine. I love it! Mine is what they call the 2004 1/2. Mid year they offered a heavier automatic tranny for it. Mine is rated 305hp and 555 ft-lbs of torque. I pull a 34' Avion triple axle that is heavier than an Airstream of the same year but a bit lighter than the last 34' Airstreams. Anyway, it grosses about 5 tons.
Running 70-75mph on the interstate I get about 12.5 mpg towing. It would probably do a lot better running 60mph, but I don't drive 60mph on I-95....as everybody else is running 80.... I get 19mpg running empty with it at 75mph, and I have gotten as high as 22mpg with it running 55-60 empty. My dad's buddy's truck is just like mine, only he got a 6-speed manual and his is a 2wd....he gets 24mpg running empty.
My buddy has a new Tundra like mentioned above with the 381hp. It's a nice truck, but torque wins the race and the Ram has gobs of it. I pulled my 34 footer over the Great Smokey Mountains and never hit a hill I could not accelerate up.
Admittedly, it is hard to find a used one with less than a million miles on it for a reasonable price. I myself wouldn't consider selling mine for less than $30K, and I don't care what Kelly or NADA says. I paid a lot more than that for mine, I am religious about the maintenance, and she looks like a brand new one seven years later.
Anyway, you'll be hard pressed to beat a Cummins diesel. My Ram is a big hulking beat with tons of power, and it gets 30% better economy than my old half ton Suburban did. Diesels rock, and Cummins diesels are like Heavy Metal if the rest are just rock and roll.
The only downside I've seen to the Cummins is the cost of entry. If you can get a deal on one, you'll not be displeased.
However, to muddy things even a bit further....when you say fulltiming, how often do you plan to move? In other words, if you'll set up in one spot for 6 months to a year and then move, you could just get a Miata and hire a guy to pull the trailer. Seriously....even Nicky and Tacey (Desi and Lucy....aka from The Long Long Trailer) were advised by the New Moon salesman to hire a pro to pull the trailer from CA to CO for them. But, if you plan to move fairly frequently, then you'll need a bigger rig than a Miata
Best of luck!