I would not open the ones in front of the AC. The wind forces on the plastic covers and lift arms could damage them. The one in the rear though should be in the slipstream of the AC unit and not subjected to the full force of the moving air.
I think the PO of our 78 drove with the vents open, they are all bent up and will not close properly. That is why I am replacing them with fantastics.
WBCCI #5501 AIR # 49
1978 Argosy 28 foot Motorhome
Wise men talk because they have something to say; fools, because they have to say something. -- Plato