If anything will help, the 303 is probably it. I'm not real optimistic, but it's easy enough to do it, so why not try? However, I think heating of the RV surface is as big or bigger factor than UV, so a UV inhibitor may have little extra value.
Decals age in sun and weather but their quality varies like most anything else. It's just another thing that manufacturers can skimp on (or not) and very hard for the buyer to tell the difference until too late. I've seen 12-15 year-old decals that still look like new and never received any extra attention. And 5 YO ones that are peeling or cracked.
