Old trailers like mine with aluminum windows
, have this problem whenever exposed to rain or a lot of moisture like heavy dew. The best way to keep maintenance to a minimum is to make sure you have a appropriate layer of fiberglass wax on your trailer.
If you have an old trailer with a worn gelcoat and little or no wax, they are spray fiberglass cleaners that take the stains off instantly. It's hard to believe but I used it when I first got my trailer and it removed them like magic, poof! But these cleaners also remove wax. So I would think it's best to keep your trailer waxed and the stains should come off easy.