The outside of your RV goes through a lot, it is always facing bad weather, the hot sun, and bad road conditions. Like metal, the paint on the outside of your vehicle will react with oxygen and start to break down. This is actually the paint itself in the process of self destruction through the action of the elements.

So, what can I do about it? Well, the auto industry has developed an answer for you. Use specially developed
soap to ward off paint oxidation. Household detergents and cleaners can actually harm the surface due to the alkaline or acidic nature of these products. Apply a quality automotive wax periodically or use the cleaner and wax combination when washing your unit to provide continuing protection.
You should also look into getting mud flaps, because they go a long way to shield your exterior from all that road debris. The splash guards that look like brushes can also be very effective when used in conjunction with mud flaps. If you regularly travel on gravel surface roads then consider installing a rubberized stone guard material on the front lower portion of your trailer. It's amaxing how fast gravel can strip the paint right off, and do damage to the fiberglass or metal of the hull.
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Repair article on Squidoo.
Why You Must Protect The Exterior of your RV