The Schrader valve is in place to fill the tanks should the engine compressor fail and the system needs to be charged to raise the coach and release the park brake , before towing.
There are one way check valves that would prevent air from the tanks being released through this valve .
The lanyards on my coach had never been routed through the frame to the passenger side wheel well , and I found them still coiled up on the top of the front tanks . Never extended by the assembly line workers OR sought out and used by the two previous owners in 11 years.
Only two tanks in the front of the chassis , one has two chambers .
I'll see if I have a diagram that shows the drain valve locations .
You could pump the brakes a couple of dozen times to insure no air in either system.