The rear shell/floor junction is a design flaw that Airstream quietly acknowledged by placing a tubular seal at this point on 2012 and newer models. You would have to separate the area to install such a seal, but externally applied sealant after removal of the trim seems to be what Airstream Service and others are doing to their Airstreams.
Zig' refers to preventive maintenance. We have a Sonin Moisture Detection Meter that I use to probe through the vinyl into the wood subfloor around the perimeter of the trailer regularly to check for moisture from leaks. That's the only way it can be done, without lifting the floor.
Searching for the culprit leak, don't look past the curved pano windows. These thin seals fail after exposure and you need to clean them out and reseal as needed, which may be every couple years. I have cleaned them, carefully taped the edges, and resealed the bottom of the curved windows after only a year of new because of a very thin separation of sealant and glass. About an hour or so.
For repair, I would call Airstream Service Center in Jackson Center, OH for their recommendations. They fix this stuff daily, and are very good at it. You are not that far away; they can give a you some advice and a repair quote.