Battery Box Rust
You should check the bottom of your exterior battery box if you have one. I found the following issue on our 2015 28ft. Flying Cloud. We traded our 2012 25 ft. FC in October of 2014-so warranty end is approaching. I contacted the dealer and Airstream customer support about the issue. I expressed my concern that the batteries may someday fall through the bottom and damage our trailer or some other vehicle. The response from customer support was that the area should be wire brushed, sanded and re-painted. I did not want to take the trailer 130 miles round-trip to have this done, so decided to do it myself.
I removed the batteries to make sure that the rust had not penetrated the interior of the box. The batteries sit in a plastic tray(more about this later). There was no rust on the interior bottom of the box.
I wire brushed and sanded the rusted area and spray painted with a rust resistant gray paint. I am satisfied with the results.
In my opinion, the cause of the issue is that the plastic tray sits flush on the bottom of the box, not allowing the collected water to escape as it should. It just sits there and eventually weeps out to the bottom surface and causes rust if the paint has weakened or was not applied correctly to begin with. There needs to be space for water(from rain or washing) to flow through the escape holes. When I next have the batteries out I will put rubber feet on the bottom of the tray to create space. Perhaps Airstream will figure this out for the future. In the meantime, you should make sure you don't have this problem.