I will add that installing the Fantastic fan in the vented opening was quite easy with the new interior shroud (square vs rounded corners to fit the Fantastic fan).
With the minimal power draw and our replacing the original lights with all LEDs, I simply snaked the wires between the ceiling and the roof and attached to the light wiring.
Orient the fan's controls for your convenience.
Opening windows and having the fan blow IN will provide direct breeze and exhaust the camper quickly. Reversing the fan to blow OUT will draw air in the windows and provide gentle (Low setting), indirect air for comfortable sleeping.
One of the BEST upgrades we made ;)
When cooking, we turn on the hood exhaust fan and make the ceiling fans vent IN, which makes very efficient exhausting of cooking odors.
Nice for breaks and lunch along the route, when air conditioning is not available (not setting up generator, etc).
We didn't have any water come in, even in heavy storms.
One more: When overnighting in Walmart parking lot and trucks are close and idling, cracking a few windows a little and having the fans blow IN provides FRESH AIR from the roof line- instead of risking exhaust and carbon monoxide from any nearby vehicles.
In short...you won't regret at least one Fantastic fan instead of just plain ceiling vents :C