I wouldn't buy new either. Unless you're legitimately wealthy, the $30,000 depreciation is a financial hit that noone needs to take.
I just finished driving everything withing a 50 mile radius and wound up getting a used truck. From what I learned in person and teased out of a few reputable online comments, it seems that:
-the Chevy Duramax is an engine that everyone acknowledges is a real beast. It tends to dominate at racing and pulling events. The problem is that Chevy trucks don't have a solid front axle and seem to have a reputation for not being anywhere near as tough as the engine. The Allison transmission is acknowledged to be superb.
-the new Cummins is not without its problems. It has not been an unqualified success and the old 5.9 liter is still the acknowledged "best engine ever" that people remember with a tear in their eye. Dodge has the poorest reputation for build quality and coach work, although I've owned Dodge trucks and they were fine. But then I owned them from before the time when truck interiors were nicer than our living rooms.
-with Ford, things get complicated. The new Power Stroke is acknowledged to be a success - many years in development and testing, it seems like they "got it right". And the Ford body/build quality is thought to be the best out there. But there is weirdness like the exhuast fluid (urea, basically) that has to be added along side the diesel fill, to keep emissions clean. That's more stuff to carry if you're really travelling. I've been told that economy is nowhere near what is advertised and winds up being basically no better than gas, and with the cost of diesel, you're actually going in reverse.
-used Ford: the 7.3 is a bulletproof engine, and there are a zillion trucks out there with it. I don't know of any designed-in flaws. The problem is that many earlier Ford trucks had other design issues that make them problematic, such as the weird front axle, or a flawed transmission (I put several transmissions in 90's era F150's), so it gets to be a sleuthing game of "which year", and you wind up looking for a very specific model year which can be a hassle. I soon gave up as there didn't seem to be one good year that I could look for. Also, they're going to have leaf springs up front which means larger tires are more likely to rub, requiring lift and spacer issues that a coil spring truck doesn't usually have. Not a big issue, but it's there. If you're running stock or stock-ish size tires, no big deal. Which are of course fine for genuine expedition type travel.
I bought a 2005 6.0 liter. The 6.0 liter engine is bullet proof in thousands of school buses all across the fruited plain, but when Ford put it in a passenger vehicle, they had to pass emissions, which meant a very weak, buggy EGR cooler system which (if over loaded and over heated, or sometimes just for no reason at all) will fail and allow coolant to get into your cylinders. The thing is, this is a very isolated, very well known problem, and there are any number of companies with solid fixes for it. Check out bulletproofdiesel.com for more info. Also, the stock oil cooler is a possible failure point. Replace with aftermarket, and you have (in my mind) the best of all worlds.
If you watch ebay, you can find (usually about once or twice a month) a really clean "first Generation" Cummins 5.9 liter with a manual transmission. If you have the cash on hand and are comfortable with an older truck with a lot of miles (sometimes not a lot, if you're ready to pay up) then that would probably be acknowledged to be the best American diesel pickup. But make sure you get a manual - the automatic from the period seems to be problematic. My original first gen Cummins I bought in 1992 was just awesomely solid. If it had been 4wd, it would never have left my cold dead fingers.
My truck has superb performance, and what's more, I have a Dana 60 front axle and coil springs up front, so I can run much bigger tires (if desired) without having to lift the truck up like Bigfoot. In 2005, they changed to the Dana 60 with coils. It's a long bed, 4 door cab, with loads of interior room, and the coils enable me to go up several tire sizes without a lift to get that long wheelbase up off the dirt. I'll drop about $2500 into it for a bullet proof EGR and oil cooler and I'll have a truck I can drive for decades.
Hope this helps. I'm not trying to sound like a knowitall, I just finished going through the whole process and it's all in the front of my mind.
Many here will disagree, or have their own perspectives, borne from far greater experience than mine.
