Have you ever discovered hundreds of tiny hard black dots on the side of your trailer that almost nothing will remove? Most likely they are artillery fungus spores. They can do considerable damage to the finish of cars, siding, RV's, etc. Just posting the info here in case you run into it. I have just gone through my second time removing them from our FC. Unfortunately good cleaner, elbow grease and time are the only things I have found that work to remove them. Bug /tar remover and typical solvents do not help any better. If you encounter it, read about it at Arillery Fungus-FAQ
before doing anything. But remove it ASAP, as it hardens and stains with age. Here is a photo of part of my latest encounter.