What to use or do?

There's not much you can do now that it's faded. What you can do the next time is use a UV blocking wax and make sure that it is reapplied regularly. Many red pigments are extremely unstable in the presence of UV, a great example of this is the seats at the stadium in Tampa Bay, they turned pink within a year. The same company that made the seats had previously made seats for the Washington Redskins without issue, it was just the specific shade (made to match their uniforms) that the Buccaneers had insisted on that didn't last. While the issue is more pronounced with red pigments a lot of trailers suffer from fading of decals and other paints due to the sun. The manufacturers aren't using automotive grade paints that are proven to last because those paints are pretty expensive and covering the square footage of a trailer would add a non-trivial cost which would be unlikely to be recouped at sales time.