[Tf175]

I had a leak at a hose fitting going to one of the suspension exhaust valves ( electric over air solenoid ) . That leak was always present , whereas your leak seems to be intermittent . I'm guessing here , but you might have a look at any solenoid that exhausts air . Your coach probably has the "six pack " valving , which should operate as my separate valves do . I can run my engine , get everything aired up , etc , supply and brake will be at 120 psi . In a few hours supply will usually be at or near zero , brake still at 100 or above . If coach is driven for several hours , the supply pressure will stay up for 2 or 3 days ( not at full pressure , but won't leak all the way down ) . It seems that this must be a valve somewhere that likes the effects of operation . Search continues ..
I also had leaking PPV ( pressure protection valve ) . This valve prioritizes air for the brakes , it's importance goes without saying .. It was not easy to access as I have to work from the ground !
As Richard says , isolate sections of the air system . Taken as a whole the system can appear bewildering , but when subsections are considered it minimizes the complexity . 
Chris