Well, they didn't last long, replaced all three roof vent covers, UV damage was incredible, the rig is only three years old and the vent covers were so degraded that one broke, (they were very brittle) when a heavy wet snow accumulated on the roof. On a side note, the windows still look new. Any body else replace them yet? My previous TT was seven years old, the vent covers were faded and cloudy, but they were still in one piece when I sold it.
Did you have any aftermarket vent covers installed over the OEM roof vents, or are you talking about the plastic vent covers that came with the trailer as part of the vent system? We had the larger vent covers installed on the roof over the Fantastic Fans that we got with our trailer and those aftermarket vent covers (MaxxAir2) are really nice. They save wear and tear on your vents and you can have the vents open even while it's raining or when you're traveling (although that allows dust and dirt to get into the camper as well). They are easy to install and only cost about $80 each.
OEM vents that came with the trailer.
For fantastic fan I recommend Fantastic ULTRA BREEZE those cover they are made specifically for Fantastic Fan and they are supposed to allow a better airflow. There is a video on youtube that show the difference in airflow with competitor.
http://www.fantasticvent.com/fan-tastic-vent-s-retail-vent-covers.html
I always stuck with what came with the trailers I've owned over the years, the cost of a Fantastic fan and the recommended covers adds up to a tidy sum,I paid $60 for new covers, (three vent lids). to replace them with the fantastic vents with the covers would be approximately $500.