Simple and easy solution.  Go to the store and buy those rubber stickers you put in the bathtub to keep from slipping. They are waterproof, and flexible. Put some on both sides of the thermarest. It will keep the pad from slipping around the floor of the tent, and keep the bag from sliding off the pad.  
Also grats on the longevity of the pad.. had my thermarest basecamp model for about 32 years as well.