Honestly, I think the dash air is rather useless. There is too much volume to cool, unless you have a curtain behind the driver and passenger seats to keep the cool air contained.
We almost always leave the dash air off and use the roof air while driving by running the generator. I think running the generator uses as much additional fuel as running the dash air.
Our model has a sliding door that separates the kitchen from the bath and bedrooms. We keep it closed and run only the front A/C. The front of the coach stays very comfortable.
'03 41' Friendship & 24' trailer with HIS '03 H-D Heritage Springer, HER '15 H-D Freewheeler Trike, and '08 Smart Car.
"Adventure is found in the journey, not the destination."