"What do they use for the cabinet carcasses, then? Sheet metal? Fiberglass? Roto-molded plastic?
Plywood is one of the best common cabinet carcass materials; it's strong, it's relatively lightweight, it's pretty much stable dimensionally, and it doesn't crack or warp the way solid boards would. With appropriate adhesive, it can withstand some dampness without swelling or falling apart the way most particle or chip board materials do. Of course, there are various qualities and grades of plywood; some is much more satisfactory than others.'
Don't know about other manufacturers but Nexus uses real {read "quality"} wood, not plywood or particleboard which others use because they are cheaper, not better. When they absorb moisture they warp and or delaminate.
All of my cabinets are solid Maple {Amish craftsmanship} and the drawers and storage areas are solid wood with heavy duty slides. After 6 years still no cracks or warping. Yes, it costs more to do it right but you end up with a better quality coach. My storage bays are made of steel.
Nexus does not use Luan/plywood in the walls or ceilings going instead with Azdell. Yes, it costs more but Azdell is a far superior, albeit more expensive product. It is stronger, lighter with a much higher R value than Luan {plywood} and will never absorb a drop of moisture. Lance and Nexus pioneered this technology years ago and only recently have some of the other manufacturers finally begun to get on board.
While we are discussing build quality, Nexus frames their coaches with low allow steel instead of wood or aluminum resulting in a 72 percent stronger coach without additional weight. Lots of variety in manufacture techniques out there. Companies like Coachhouse, Phoenix Cruiser, Born Free, Lazy Daze, Nexus and yes, some of the higher end Winnebago lines continue to find better ways to build quality coaches.
The 30 to 40 year old process of framing with 2 X 4's and aluminum will eventually go the way of rubber roofs {as fiberglass roofs with one piece solid end caps or even one piece fiberglass coach bodies render them obsolete}. The differences quickly become evident when you start looking deeply at how a motorhome is actually constructed and finding plywood is nothing to write home about.
As always.... Opinions and YMMV.