Phil,
You have a point when you say our RV's aren't built for off-road use and such use requires different construction.
My point in this discussion is that any ordinary C can twist. Only a matter of degree.
I mentioned our Jayco's STEEL house frame. That seems to be why that very basic 31-ft, no slides, limited features inside, no basement, weighs so doggone much.
On the truck scales, 4500 Front and 9000 Rear, loaded and occupied for travel, full fuel and water, empty waste tanks. That was a big part of my decision to upgrade the front axle from 2002 to 2012.