Why wood roof trusses?

Fairly certain that Keystone still uses wood over the top of those aluminum trusses and wood on the subfloor as well. Perhaps the trusses won't rot, but the wood roof structure will rot, and that water will eventually find the wood floor and rot that out too. That will cause the aluminum structure to sag, requiring that to be replaced too. The same problems will occur, wood or aluminum. The wood is cheaper to fix though and doesn't require special skills or tools.

Until they make a trailer that is completely impervious to water infiltration, its a moot point. Azdel walls are a great start... but wooden roof sheathing and subfloor will still absorb water and eventually rot or grow mold.