The quote below is from the W&P website. Since this plant is set up to deal with wood construction, I would bet that it has some sort of wooden frame of 1 1/2" dimension lumber( 2x4 cut in half) stapled together with all cut outs, doors windows etc framed out, then fibreglass is bonded to the outside, some sort of finished material(paneling) is bonded on the inside. Very common RV construction technique, inexpensive but has the fatal flaw that all wood construction has. ROT. The website is woefully inadequate about details, I suspect for good reason.
They give plant tours, one of these days I am going to get over there.
2″ Laminated Walls on Ultra and LE Series (N/A on FRP models)