If you are going to buy a vent cover, do yourself a big favor and install a Maxxair Turbo Fan. Installs right over the existing vent same as a plain vent cover. Don't buy anything with a rain sensor.
The fan is 'outside' the RV so you don't even hear it. If you have 2 roof vents if you turn that puppy on it will recycle the air in your RV in seconds like a wind tunnel! Have only one vent close all the windows but one and same wind tunnel cooling!
With this fan I hardly have to use my ac anymore. And of course you can leave it open all the time.

Definitely get smoke colored roof vent covers. I had one of my 'smoke colored' factory vent covers get ripped off in a storm and mobile repair replaced it with a white one.
Later that year during summer I was walking thru the coach and all of a sudden I felt a sudden wall of heat......I was standing under the replaced 'white' vent cover. Boy had no idea how much heat comes thru those white ones! Replaced it that day with a smoke colored one. Big difference.