Well, if plywood is to be found anywhere in a Class C's structure .... it certainly ought to be marine grade plywood .... but good luck finding any Class C with that in it.
I prefer aluminum structure everywhere up high and steel structure everywhere down low in order to keep a Class C's center of gravity low.
However there is a caveat regarding use of steel or wood in an RV's coach structure - I don't know which is worse in case of water seepage here and there - steel that rusts or wood that rots.
I guess that high grade oxidation-resistant aluminum everywhere for the structure would be the best compromise material.