You could write a book on this subject! Its really not fair to compare a domestic truck to a mog, they are entirely different beasts made for different purposes. The gas milage in a diesel mog is not too bad, I get almost 15mpg from the double cab 416 when driving it flat out. The bigger mogs such as the 1300's gets a little less and the newer more powerful turbo diesel mogs may be as low as 10mpg, my camper mog also gets a little less at about 12-13mpg fully loaded. You can carry a rediculous amount of fuel, my camper has nearly 200 gallons with the tanks and the jerry cans, thats well over 2000 miles or about 1800 miles with just the fuel tanks. Just the weight of the fuel is more than most domestic trucks can carry.
Most people who travel in a mog view the off road capability as an insurance policy, when the vehicle is your lifeline you don't want to be operating near its limits or risk breaking it or getting it stuck. On the other hand, there is no point in using a mog and then sacrificing its offroad ability, there are much better trucks available if you are going to do that. The fuso seems to be a common choice, its faster,quieter, has creature comforts such as AC and probably a little better gas milage. In general I don't think most people would be willing to travel in a mog for thousands of miles, it takes a special breed to drive everywhere at mog speed - 55-60 on the flats and as low as 30 on a good hill. In addition mogs are high maintainence machines, you do have to keep up on the basic stuff such as oil changes (and there is a lot of oil) and lubing.
In return you get a truck that can drive fully loaded over the worst roads in the world for 250,000 miles and barely notice. 45 degree hills, 3+ feet of water or pot holes and rocks 18 inches across can be driven over without a huge amount of skill. Technical driving is fun but when you do it on purpose but it gets tedious quickly. When you are in the middle of nowhere you can't do all the driving, my wife doesn't like the technical stuff but she is happy to point the mog in the correct direction and drive over whatever is in the way. I relax much better when she is driving because I know the truck will deal with most things - you have to be careful to not confuse the mogs ability as your own ability! Having to use technical driving skills for 100 miles of bad road would quickly wear thin.
Travel mogs are typically not mechaniclly modified, you would be a fool to mess with one. You have to think of a mog as a complete system, many people have put V8's in a 404 to get more power only to break the drive train. People put mog axles under jeeps and break them and then say the axles are crap but you never hear of a mog breaking its own axles even though the mog is 2x heavier - its all about the complete system. Fully locking diffs are great but they don't make any sense if a single half shaft can't handle the forces when its driving the only wheel with traction, the truck is fully loaded and the terrain is nasty. This is why a lot of US trucks break because they are not designed as a full system. Most US trucks need heavy mods as soon as they leave the factory do even do what the commercials show them doing, just look at the mods on this forum.
To answer your original question, if you are staying somewhat local and you have a trail ready truck you will get pretty much anywhere a mog will get, in fact in wooded areas you may do better due to being smaller. Look no further than the people on this forum they have been all over the place in domestic trucks.
My mogs are always available for people to test drive, on road or off road.
Rob