Could be the bag it's self is leaking, or depending on how the bags are controlled, a ride height control valve leaking, or an air supply hose leak.
There are many check valves involved in the system , to isolate, the air suspension from the air brakes and the bags from one and other.
If you are under the coach checking for leaks , block the frame of the coach to avoid injury.
Bags and lines can be checked with a spray bottle of soapy water.