I have a 1999 (OCT 99) Dutchman alum sided trailer. I have every year in the spring washed it to remove the black streaks from the sides. After a good cleaning, I have hand polished the entire outside with a Polish called
"NU-FINISH". My siding today is still in pretty good shape considering the age of the TT. It is a good two day job to polish it completely (working 4-5 hours each day) but it is worth it. Follow the directions on the NU-FINISH container. You have to go back, looking down the sides to see areas that are missed. When you are finished, you will admire your new looking again RV.
The NU-FINISH polish (I understand) was developed for application to be applied to the surface of the of the space rockets while sitting on the launch pads being prepped for launch. Seems that the polish prevented the salt from the the air at the Florida launch site from adhering to the surface. (a fly even has a problem landing on my car because it is so slick)
All I know it works for me here in the rough Florida weather.