Normal air sucks in a mixture of recirculated air (From inside the cab) and outside air.. this is normally the best option.
MAX air, on a motor home is generally useless, closes the outside air intake and sucks air only from the cab inlet.. On most cars you hear this as an increase in noise which you assume means an increase in fan speed, it does not. there is no increase in fan speed.. It is simply an increase in vortex noise at the air inlet.
On a motor home the cab air inlet is often much larger and does not produce vortex noise,, On mine it is nearly a square foot of gridwork.