It is a vinyl wallpaper over 1/8" plywood, but you won't find just the paper somewhere. It comes already attached to the panel. You can get it from an RV dealer, especially if you don't need a full sheet. A lot of times the manufacturers will crate up parts with scrap pieces of paneling, so you can get a decent size piece to use if they save them. The parts are usually crated with the nice side in for that purpose, to reuse.
You probably won't find anything to match though, there hasn't been blue/gray/white wallboard in a long time.
2007 T-286SR Cherry/Granola, #6236, original owner, current mileage: 9467.8 (as of 5/26/19)
1997 T-2653 Blue Denim, #5471
1979 12 1/2' MC, Beige & Avocado, #4639
Past Sunlines: '97 T-2653 #5089, '94 T-2251, '86 T-1550, '94 T-2363, '98 T-270SR