Sleepy,
These things are stable because most of the box you see is air. The same reason an empty box truck is stable. These campers have the engine, transmission, heavy frame, tanks, batteries, everything heavy, down low. All that is up high is food in cabinets and air. Just because it looks top heavy, doesn't mean it is. The frames on these commercial trucks weigh considerably more than your everyday 1 ton as does the running gear. That helps out a lot.
If you've never driven a Fuso or equivalent with a load, you can't understand how much better suited they are to this than a normal 3/4 or 1 ton truck.
