We installed the Maxx window covers last year. They work very well to keep the rain out. We installed one on the window at the foot of the bed, the other on the window over the side dinette, as I don't like to leave the window over my head open at night. This way we still get a "cross breeze," but not across my head.
We also installed the Maxx hood over the FanTastic fan (the more expensive one, which allows the fan to open fully, and run on automatic...)
My observations, after a season with more rain than usual:
the window vents work wonderfully. Not even a drip inside. They do, however, cut down on the amount of air entering the trailer compared to windows
with no vents. When it's not raining, I tend to open the windows
Ditto, and moreso, with the hood over the fan. I love it in the rain, and hate it when it's not. My guess is that it cuts airflow by 50-70%. The fan can make up for it, by running at a higher speed, but at the cost of many amp hours per night. Seriously, when camped for a week or more with a good forecast, I've considered climbing up and flipping it over on the "hinge". Never did, probably never will, but in warm temps, the hood just really makes nights much more uncomfortable unless the fan runs on a higher (and therefore noisier) speed.
All in all, in rainy weather, the mods were definitely worth the rain-free ventilation, as we rarely camp with hookups, and therefore rarely run the AC.
My two cents.