Any progress? The warning buzzer is for low air not the battery switch. turning off the battery switch just shuts power off. The warning will not go off until after you start it and it builds up enough air to inflate the bags and release the parking brake and you have enough air to safely operate the brakes.
Try this, turn all battery switches to on. Can you start the generator? I would start the generator and let it charge the batteries for a few minutes. You need to run the generator Monthly any way. The couch should default to the generator this way you know you have a good power source.
Let me know what you find.
Bill