Material Backer Behind Filon Pin Box Underhang Area

Looking for some help on finding the material that is behind the filon in the underhung area around pin box and front storage compartment. I pulled off the filon and what apears to be some sort of cardboard material? It has a paper over the cardboard type material and this is behind the filon. I have found the filon to purchase to put back on, but not having any luck with what is behind the filon. I have an 08 Bighorn that experienced a frame chassis issue under the bedroom floor that I am repairing. Any help from the group is greatly appreciated. Here is a picture that you can kind of see the material I am asking about.