The only dumb question is the one that isn't asked.
I think the general answer to your question is metal. Here's a portion of the structural description from the 2006 brochure for the Ultra LE --
"Welded aluminum perimeter frame."
"Laminated steel frame floor w/metal underbelly."
That said, there's plenty of wood used in the construction, too, IMO.
You can find the 2006 brochure
here.