[rheavn]

Every air connection fitting, air valve, air tank, suspension valves and air dryers are suspect.  They all have the potential to leak.

I have replaced air dryers, the air pressure protection valve, an air tank, all push in type connectors with DOT compression fittings, both slide shut off valves and a few things I've probably forgot.  Almost all basement compartments, under the coach and inside the coach have potential leak points.  

My opinion is that the key to finding leaks is to be organized and isolate individual circuits.  Verifying that an air circuit is NOT leaking is just as important as finding a leaking circuit.  My favorite tool is the leak down tester as it will tell you if a line is leaking--or not.  Then if you have a leak you have to find it and the ultrasonic air leak tester and soapy water become useful.

The only thing I have NOT found leaking is an air line itself, but the potential is there if chafing occurs.

Yes I have two air toilets.

Search for the air circuit schematics that I have posted and it will help you make sense of spaghetti.

Good luck in your search................................