This is an interesting thread. I think the causes are far deeper than is being discussed. Coming from the truck camper forum, and dealing with an off-road XTC, I see some basic and similar flaws to body-on-frame RV's for off-road use. It really doesn't matter if the frame is wood or metal if the box is bolted directly to the truck frame without some form of 'pivoting' ability. What's that? When the truck frame flexes/tweaks/bends/twists/racks, as it most certainly will if you go off-road, following the twisting suspension, the C body will will try to follow the twisting of the frame and slowly and inexorably try to pull the C's body apart. The mfgrs of class C's in this country are counting on you never going off-road, so they don't have to invest the time and resources to a pivoting sub-frame. European and a few North American truck campers or class C's have a sub-frame that has either one end solidly attached to the frame and the other end connected a big center pivot bearing to mitigate the body of the C twisting. Another way to attach the camper part to the truck frame is an elongated diamond shape, 4 point frame connection. The solid connection is side-to-side across the center with the two big bearing surfaces at either end to go with the twisting frame and stay flat and square. XP Camper builds their own pivoting aluminum flat bed to go under their state-of-the-art camper which is bolted to truck frame on a 3-point pivot system.
Of course all this came from my Dodge 2500 experience and since I do not have a 3 or 4 point pivoting subframe, I rely on adjusting the tie-downs keeping the rear most ties very loose allowing the rear of the TC to 'float' on either side when the frame twists. Here is a link to an interview I gave concerning the twisting and its consequences while going off road: http://www.rv.net/forum/index.cfm/fuseaction/thread/tid/27830086.cfm
regards, as always, jefe
