when the AC is on, the vents are closed. My AC is more efficient that way.
By way of explanation I have a class C with 3 slides and only one AC unit. There are enough small air gaps that even though hot air would rise out of the vents when the AC is on, the reality is that warm air is also drawn in through the gaps. With the AC on and vents closed, the AC doesn't work as hard and will cycle on/off. With vents open, for the same thermostat setting, the AC will stay on far longer.
I have MaxAir vent covers though I would fit Camco Aero Flo if they weren't already there as the Aero Flos are easier to clean.