[Richard]

Thank you for adding that you monitored the brake pressure during your trip.
That makes me feel a little better. If you had been traveling in mountainous areas then long descents could have been more problematic.

Since you are parked it may be easier to determine the big THREE.
Leak or governor or compressor. When you use shop air and disconnect the shop air, what happens? If th coach holds air then massive leak is not the issue. If the pressures on the guages drop rapidly then obviously you have a huge leak. If the brake tank guages drop at the same rate as the supply gauge then your leak is in the brake system . If the brake tanks hold but not the supply, then at least you know the leak is not in the brake system

If the pressures hold when using shop air then the next test is to disconnect shop air, bleed the air pressure down, then crank the engine. At idle, what is the max pressure the system will build after five minutes ? If you increase to 1500 rpm what is the max pressure the system will build?

If the max pressure the engine comp will build does not change with RPM then the governor may be a problem.
However if the max pressure changes with RPM then the compressor is a likely problem