Boy, You’d think after 30 plus years Airstream would have figured out how to seal up the rear ends of these things.
Both of the Vintage rigs I’ve owned have had similar damage.
Now that you’ve cleaned and removed the loose ply, you’re correct in that you need to stabilize the area.
I’ve used Git-Rot in the past with good results. BoatLIFE | How to use "Git"-Rot
This is a liquid epoxy that will strengthen the damaged plywood before filling the remainder with epoxy. Dry out the ply as good as you can.
I don’t know precisely what AS is using for insulation on the newer trailers but for Vintage rigs we recommend removing the Fiberglass insulation from under the floor to help prevent moisture wicking into the new repair.