Our experience has been that the dash air helps the driver and front passenger have nice cool air on their face but even with a vent open in the back to pull it back further...it did little to nothing @ temps of 90+ especially in high humidity area.
We found the dash air did not draw down engine power enough to be noticed. The genny will use approx. 1/2 gallon of fuel per hour.
Our dash air quit and repair guy says it will cost hundreds to fix it so...we just use the roof AC units.
The best thing to do is try it and see what works for your comfort level.