That means the walls & roof use aluminum framing. It makes for a better structure assuming other good practices are in place. Wood expands and contracts with the change in seasons which is harder on caulk-work. And as you pointed out, wood can hold water and rot when gotten wet.
Regarding floors, most if not all RV manufactures use steel beams with wood over it. Low end models get chip board. The better ones use marine plywood. Some brands like Phoenix Cruiser get a thick plastic corregated under-belly protective barrier with foam block insulation in between. Admittedly I am not familiar with every flavor construction method in the making of floors, walls, and roof.
Phoenix Cruisers Get Marine Plywood, Foam Block Insulation In Between Steel Beams, Thick Corregated Plastic Underbelly
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