The skinner valves shut off the air flow to and from the air spring on each side when the ignition is off to help prevent bleed down. The compressor is in the exterior rear storage compartment. It may be behind a false wall in the compartment. If you drain the air tank and turn the ignition key to "on" but don't start the engine you should be able to hear the compressor at the rear of the coach.
"Socialist governments traditionally do make a financial mess. They always run out of other people's money. It's quite a characteristic of them."
~ Margaret Thatcher ~