What you describe is called aluminum filiform corrosion. My 2001 19-ft Bambi has this as well. I first noticed it in 2001, a year after taking delivery of my Bambi, and I've seen it on many Airstreams of my model year and newer.
For your reference, an outstanding journal site on the technical aspects on filform corrosion is from the Journal of Corrosion Science and Engineering: http://www2.umist.ac.uk/corrosion/JC...r36/v2p36.html
complete with pictures and a thorough discussion of the process.
The corrosion often, but not always, begins near a cut in the skin, for instance, at the edge of a panel or at a rivet hole. But I have also found this corrosion starting on a panel surface with no hole or edge.
Once it starts, there are two ways to deal with this corrosion that I know. Either I have to strip away the plastic coat surface from the entire affected area and a small perimeter around it, and then keep this polished, or I apply a chemical developed to deal with this.
It is important to note that filiform corrosion does not originate from the action of dissimilar metals, and is not going to cause a structural failure or problem. It does cause a cosmetic problem, as you are well aware.
The corrosion begins from the action of water and chlorides found in the air gaining entry between the plastic coat and the aluminum, and can proceed quite rapidly. It can start in conditions where a surface has never been wetted, only exposed to humidity.
To stop the corrosion when it has begun, for the past 14 months I've been using a chemical called Corrosion Block, also known as ACF-50, made by Lear Chemical Company, Mississauga, Ont. See their site at: http://www.learchem.com/five.htm
. This was developed for the aircraft industry, and it has been available for other uses as well.
The aviation product is ACF-50, for Marine and industrial use the tradename is "Corrosion Block" and for automotive use the tradename is "Rustblock." The technical specification is in a PDF: http://www.learchem.com/images/Tech%20Spec.pdf
Yesterday I did a close inspection of my Bambi, and found several areas where filiform corrosion was starting again along a previous track. I reapplied the ACF-50 again to all known sites yesterday after that. I am pleased with how this product is working. But I have to keep up on it.
I've taken literally hundreds of closeup digital pictures documenting this problem on my trailer to understand how the corrosion has progressed. I've attached an example from my Bambi.
Contact me if I can assist further.