No matter how internal wall structures might be built with slide-out holes in them, you just cannot get the shear strength that an integrated flat piece of stock - with no or minimum square holes in it - has. It's strength of materials and how they're used in play here - no way around it.
Even with our stick house - one of the main criteria for qualifying for our earthquake insurance is "are the home's living area walls and foundation crib walls made up of closely nailed intact large pieces (i.e. plywood) of material?". I figure if maximum shear strength is required to keep a stick house in it's original shape during earthquakes, then probably it's required to keep the living area of an RV in it's original shape when tipped/bumped/twisted during off-pavement travel. These shear strength considerations are of course in addition to any isolation of the coach area that might be done in an RV intended for occasional or frequent off-pavement use.
Design of RV's is just like design of any other product - there are good enough designs out there and optimized designs out there. It depends on how much you want to trust someones else's (even an engineer's) "good enough" skills and judgement versus your own "good enough" common sense. This assumes one cannot afford an optimized design in the first place.
With only a modest budget and still wanting some reasonable size and comfort when used intermittently off-pavement ... our version of good enough required this: A small Class C with no slides for best wall shear strength, and also on the optional E450 chassis - so as to gain the additional frame rigidity to help with twist (since the coach box is not isolated) that you get with small Class C weight from the E450's thicker frame steel - over that used in the the E350's frame. FWIW, I don't know about frame steel thickness differences between the Chevy 3500 and 4500.
