Hi RAMTOUGH,
My Air Stream service provider here in OK City told me that there are 2 or 3 ways to deal with the problem. Short of what they told me, what I have done is to wash the outside with something like Castrol Superclean or an equal product. It takes the black deposits off and makes it look like new, but rinse it VERY well. Air streams have a coating called "Plasticoat" applied at the time of manufacture. It tends to wear away on the upper surfaces. My '70 model is starting to need attention, however if I clean it every 2 years as I stated above it looks good. One way they recommended to deal with it is to polish the trailer with a good Carnuba base wax. I do not have that much time or energy. Another way is to recoat it with "plasticoat", which is a little expensive but lasts for years. Since mine is only bad on the top 1/3 of the trailer, they told me that I could have the top painted with a white paint or roof coating. That would help the A/C to work easier and cure the bare metal problem. Hope that helps.