I wouldn't worry too much if the vent had a rain sensor or not, other than you want your money's worth on what you bought.
You should have the external covers installed as soon as possible over your vents, anyway, to slow up or eliminate the sun's UV ruining of your new vents. Also, if you're ever away from your RV when heavy rains start and the "automatic" vent closing feature of your vents should just happen to pick that time to not operate correctly (all mechnical things can fail) ... you could possibly have a real wet mess inside the RV.
Also, if you ever still want interior air ventilation during high winds - having the external vent covers over the vents allow you to have the vents open without the winds causing constant vent rattling or worse yet ... cracking or ripping off the vents!