I have 3 roof vents also. Covered only 2. And only 1 has fan.
1. Bedroom - only a maxxair cover, no fan.
2. Bathroom - NO cover. You don't want to leave the roof vent open in the bathroom when traveling.
3. Cabover - Maxxair Turbo. Fan is OUTSIDE the MH. Thermostat, reverse fan and speed controls.
You turn that fan on exhaust up front and it sucks the air from the bedroom and creates a tornado of a fresh air wind affect through out the 'entire' inside of the MH! I rarely use the ac since installing the fan this way.
It will clean out the air in the MH in seconds.