There is a lot of good advice from all kinds of folks in these replies.
We r driving a 11,000 kg Unimog with camper - so we have some experience with big vehicles. You can read about vehicle and trip at whiteacorn.com
Don't be put off US vehicles. The advice from others is correct. No vehicle is easily repairable EVERWHERE.
If you break down seriously you will have to source parts no matter what you are driving.
When thinking about repairs on the road think first about general service parts like filters, belts etc.
Then think about tires, wheels, springs, shocks - the stuff that bad roads destroy.
Surprisingly tires can be a real hassle. So if you are really worried about "repairability" don't go too exotic.
20" wheels and standard sizes like 20x13 20x14
22.5 with standard sizes 295/75R22.5 385/65R22.5 - these later tires are on 1/2 the semi fleet throughout europe and asia
Engines and transmissions are generally pretty robust. You should ensure those are good before you leave home.
CAT and Cummins engines are somewhat understood most places and in the US one can get trucks with Mercedes engines.
In Iran we saw lots of Freightliner and Mack trucks.
Rules I would follow even more strictly given my experiences
1. make sure the truck is NOT overloaded
2. don't modify the truck - you wont get it right the first time and trying again in a place like Mongolia is not fun
3. learn as much as you can about the known weak points of your truck - talk to mechanics that work you brand/model of truck
Keep you height under 3.5 meters. Our is 4 meters and it requires constant observation so as not to hit trees, and believe it or not building balconies in places like Greece.
Big and/or old vehicles are a hassle in Europe full stop.
Austria freeway tolls - 28cents per KM for ANY vehicle over 7.5 T and older than 2003
Germany - vehicles older that 2003 cannot get emission approval stickers to enter most cities
Germany - trucks are not allowed on the autobahns on Sundays - you might get away with it as a motorhome but not is you are over 12 T
Generally if you make you vehicle look like a bus rather than a truck life will be easier in many places. EG Road taxes entering Turkmenistan 13 cents/km for truck 9cents/km for a bus. They have no such category as motorhome. If it looks like a truck it is a truck.
You might like to look at this site - these guys try to keep track of expedition vehicles for sale
http://www.expedition-trucks.com/brokers
and they might be able to give you some advice on how to register an expedition vehicle. I know one of them registered theirs
in the UAE (United Arab Eirates).