When I was a Coach tech at a RV dealership, I would get this a few times. I always used a can of 3M spray adhesive, the clear kind, and I can't remember the number, but it might have been 88. Check behind me.
Anyway, take a pointed xacto blade and make a small slice at the edge of the sag. Use the adhesive with a tube, like the one that comes with WD-40. Spray and move the tube to make a full circle. Remove the tube, and push the fabric from the outside to the middle. Being careful not to "push" the fabric, you'll wind up with a big knot in the middle.
Repeat this until you go all around, and have it all reglued and attached to the luan above the fabric. The small slits you make should be unnoticeable when you're done. Be careful to just push the fabric up, don't ask how I know that. hahahah
If you use any other glue, than the clear, it could show up in a month or so. The other that is worth repeating, a sharp razor blade to make the incisions.
Tank Thomas, USAF Ret.
There are no secrets to success. It's the result of preparation, hard work and learning from mistakes.