The guy that I sold my 1996 21'
Sovereign to said the headliner in it started sagging also. He said what he did was install strips of bamboo. He had them fan out at the front end cap and installed them crosswise from the front fan to the back end.
I read on a post here sometime back that the problem was due to a government mandate about reducing emissions and Airstream went to a water based adhesive that did not hold up over time. I believe the adhesive was failing, not the foam backing on the trim material. I was told that if you don't get all the old adhesive off, just putting 3M spray on it was only a temporary solution.