@BortEdwards Yep, that's shelf liner, sold in a roll at Walmart, Lowes, dollar stores, etc. It's extremely handy!
I actually put dots of seam grip on both sides of my sleeping pad, not my tent. Now not only does it stay in place, so does my sleeping bag/quilt! I was literally slipping off my pad and would wake up scrunched against the wall of my tent. lol