I had that happen on the travel trailer I had before I got our 5th wheel. I bought new fenders but they're very expensive for what they are. When I installed the new ones, I put a dab of color matching silicone caulk over the screws (I know Silicone is a bad word in the RV world but it's a plastic fender that gets replaced if it breaks so no harm/no foul). It did the trick. No issues from then on. It was one of the first things I did to our 5th wheel when we got it. It survived the first year travels and winter storage just fine.