We use it on our new Airstream and like it very much. Also use it on the front window rock guard because the bugs and road grime come off easier. It is supposed to contain a glass component that reflects UV light, which if true is a good thing.
If you have never polished the trailer you can put it right on (after washing). Otherwise I believe you are supposed to remove the old polish before applying it so it can bond to the Airstream finish. The container or website should give instructions.
We first found it at the Airstream Service Center where they offer application of it as part of their service offerings, but the labor cost is not cheap. You can also order it from the Airstream Store.