The rain sensor works well also but if you intend to cover it you can save some money by buying the one with out the sensor.
If I had to do over (our Fantastic Fan was factory installled), I would not get the rain sensor and put a cover over it. This is for two reasons, first the rain sensor is super sensitive and the slightest moisture will close the vent. Second, it is very desirable to be able to run the fan, with the other covered vents (our trailer has three total) open while it's raining to move some air and pull the moisture out.
2004 Dodge Ram 2500 Club Cab 4x4 CTD
2007 Sunline Solaris T-276-SR: Misty Blue/Cherry
2001 Dodge Ram 1500 Reg Cab 4x4 OFF-ROAD