We had to replace 2 vent covers. One was covered with the Maxxair and the other was not. Both became very brittle due to the sun. I did put a Maxxair cover on the one that was not covered previously.
I think the covers used by Keystone are simply cheap. The two I replaced were both made in China and came from two different dealers.
Keystone is probably not the only company using poor quality covers.
