[Johnc]

Regarding the above, I find it hard to believe that the small 110v and 12v compressors provide air for the brakes and air bags. Perhaps the air bags only when parked. Can someone shed some light on this?

Your high capacity engine driven air compressor provides the air for brakes, air bags, etc. when the coach is on the road. The spring brakes keep your drive and tag axle brakes engaged when you are parked, and if you have your air leaks under control, the air bags need very little replenishment while parked