I heard back from Jayco but they won't come right out and say what the joists and rafters are made of. Almost like they are trying to keep it a secret for some reason? This seemed odd to me because the maker of my current rig went on and on about how their floors/roof were made and even had cutout samples of each to show during their sales pitch.
From the few videos I can find online it looks like it is 2x3 wood nailed together with drop in fiberglass insulation and plywood on top for the floor. The roof also looks like 2 x 3 wood connected together with nail plates and drop in fiberglass with plywood on top. Should be sturdy, I'm just concerned about longevity and wood rot.