[hypoxia]

Here is a tremendously over simplified air system explanation. All air supplied by the 120V compressor and engine driven compressor goes to the brake system first. The brake system consists of two isolated tanks. Those tanks are isolated by the Pressure Protection Valve. If there is zero pressure in any system, all air will go to the brake tanks UNTIL the tanks reach approximately 60 psi, then air will flow into both the brake tank system and the general SUPPLY system.

Everything but the brakes receives air from the Supply system. That includes travel mode, level mode, generator air bags, doors, and potties.

FWIW, The early coaches are apparently plumbed differently than the later models.  My 2014 model 2020P 120VAC compressor air is not plumbed to the main tanks.  That's a subject for a different thread.