Redwood does burn, which is why I said it was fire resistant, not fire proof. Kiln Dried redwood is pretty strong. Strong enough for a camper least ways on my build. With all the corners 4x4 with ripped 2x2 notch, it was stronger than two 2x3's nailed together. I was able to add a lot more cross bracing in that notch, which really added strength. We never saw a bug after and it smelled wonderful!! I used dowels and a high quality wood glue to all my joints as well. Much of the inside over cab was lined with redwood fence boards. Rustic and again, the smell was great. The boards added more sheer strength too.
The foam boards also add some structural integrity over fluffy fiberglass batting.