As David says, the compressor found under the curbside front end helps keep the bags balanced while parked and leveled. But it won't work if you manually level with the keypad; the pump is tied into the Auto-Level system, which is the button on your air-leveling keypad that you would most often use at campsites. If one corner of the coach drifts down a bit over time, perhaps hours or perhaps days, a level sensor in the controller box on the ceiling of your electric bay tells the computer to kick on the compressor, opening the valve to the low bag, and releveling things, usually within a short time.
But the compressor is not mounted in an ideal, protected spot, and often suffers from dirt or other damage that causes it to fail. Or it's electrical or air lines can get compromised so it doesn't work as intended. Just be aware of that. When it's working right, you may hear it run every now and then while staying at park or campsite. If the coach goes noticeably askew over some time, after initially using AutoLevel and without hearing the pump during that time, it will need attention, repair, or replacement with preferably a ViAir quality unit.
Joel
