The material used is
Coroplast. That link will give you the distributors in your area. It's reportedly expensive. I've heard that Home Depot and probably Lowes can special order it. There is some special tape they use on it as well. Call Coroplast and get the product id for RV underbellies as well as the part number for the tape.
The best insulation is rigid foam board, the 2 inch material. You can get that at Home Depot/Lowes as well. The RV manufacturers use fiberglass insulation which is fine until it gets wet when it is no longer and good. The rigid foam survives a water leak with no problems.
BK