Aux air
AJ: I am sure many others will chime in on this, but to answer your question, I will give you a run down on my bus as to the aux air supply: It supplies the bellows for the (2) fan belt, (1)air conditionion compressor, 4 air bags for the gen set, 2 pocket doors, bed lift, entry air door lock, drivers seat ride and bolsters, 2 dump valves, 1 fresh water valve, floor slide, front louver, and probably one or two items I have forgotten. Normally the aux air controls the leveling but must have assistance from the engine compressor on my bus to assist in the lifting process. Even the rear tag gets involved.
The problem with your compressor failing to shut off sounds like the pressure switch usually located in the compartment under the drivers seat. Generally a fairly simple deal to take the old one off and install a new one if you are going to the converter's shop. The cycling on and off about every 3 hours indicates you have a leak, but not severe. Don't worry it will get worse over time and will really surface.
I had a terrible time finding my worst leak with all the accessories involved, but turned out to be the bolster on the drivers seat.
Roger that!
2008 Liberty DS XL2
2023 Denali Ultimate
My 6th Prevost