OK, I just have to weigh in, because this is America! Having driven many many miles in loaded trucks, both single rear wheel and dually versions, I can honestly say, it depends. As HDR alluded to, some duallys can get you in a bind due to width. But in soft stuff that extra width can be an advantage also. I once drove a dually van towing a trailer right through a sand bog that our other single rear wheel van not towing a trailer almost got stuck in. Empty duallys can also be a little bit of a handful on ice, but by then you had better be going slow in a single rear wheel truck or you'll be in a ditch anyway.
As for the Turtle V truck, I'm actually friends with Gary and Monika and have traveled with them. Doing what most on this forum are doing, and comparing it to them, is not really a fair comparison. They switched to singles for a couple of reasons - They travel internationally on very remote roads for long periods of time. There are very limited size tires available for the 19.5 inch rims that came on their truck, and they needed something taller to get through the river fords etc they expect to see. Also, many of the tracks are very tight and they needed their truck to be narrower - most duallys approach 8' wide and their truck is only 7' wide - 6'' on each side may not seem like much, but it makes a difference some of the places they go. They also need a tire that is available anywhere in the world - 17, 18, and 19.5 inch rims are not found in places like Mongolia, or even much of South America. But the tire they chose also needed to support the almost 15,000 lb truck. So they went to custom made 20'' rims with Michelin 40'' tall XZL military tires.
If you plan on traveling like they do, shipping your truck all over the world and driving through jungles you might want to consider doing as they did, but you better have deep pockets, or a sponsor. If you plan on traveling in the American West, in the deserts and the occasional mountain forest road, a dually will give you much greater capacity and be more stable. My current truck is a dually, after driving my previous 3/4 ton single rear wheel truck around overloaded all the time. The dually handles the weight so much better, and with it loaded I actually find it seems to do better on the ice than my previous truck. A couple weeks ago I drove on ice from Denver to Colorado Springs, passing numerous accidents and roll overs, and never felt the need to put it in 4 wheel drive.
