[Fulltiming]

The doubler is a stand alone unit that gets air from the coach, then increases the pressure which is adjustable by the pressure regulator valve. There is an intake line (comes off your air system) and an exhaust line which will have a lower air flow but at a higher pressure than the intake line. It is only used for airing up the tires as nothing else needs higher pressure than the regulated coach system which maxes out at about 120-125 psi.

I can't read the pressure gauge on the red line in the photo so I don't know if that is the intake line or the exhaust line. Can you determine where the other end of the red air hose goes?