Acryl-R Seam Sealer by Schnee-Moorehead, Inc, is another weapon you need in your leak battles. I think this is the stuff Airstream uses to seal seams.
I had a leak causing a wet floor at my curb side storage compartment, left side as you open the door. Like Becky said, soft plywood, ugh. So I ran my leak detector test. I rigged a garden hose with shower spray clamped to my step ladder and started squirting the compartment door. 30 minutes of water flow and no leak. Then I went 2 feet higher and repeated the test, no leak. Then I went to the bedroom window, sprayed for 30 minutes, still no leak. So I set up and sprayed above the bedroom window, but below the awning mount, no leak. Next spot I showered with water was the area above the awning and their she was. Water started coming in on the floor.
I used the Acryl-R and sealed the seams where the rear end cap sheets join the roof and side skins. I also sealed the vertical seams all the way down past the window. Acryl-R is a "thin" caulk that tends to run into the seam. The application tip is very small, and you can "squirt" it into the seam.
I won this small battle in the leak war. There are Airstreams that leak, and Airstreams that are gonna leak soon. It's part of the "fun" of owning one. Something to talk about on the caravan.
Add Acryl-R to your weapons stockpile. I got mine at the Airstream store.