YEA, I FOUND IT
It is in the air door claw, somewhere between the red valve under the dash and the claw itself at the door.
I just come back from a drive, and the air pressure stays up forever with the red valve off. But leaks down in 3 minutes with it up. So after coffee, it's back to the bus with the soap bubbles. I hope the leak is somewhere accessible, because the PO had the stairs and the wall on both sides of it covered in ceramic tiles, blocking access to the claw, below the pass chair. So if I am lucky, it will be under the dash or in the spare tire comp. Will see.