If you have a "manifold" at the rear of your coach and it has service ports (plugs in them right now) that is the idea spot to "tap" in -- take out the plug and put in your tap. That is what I did for the hookup of my supplemental brake system. Roadmaster actually gives you the tap in plug to use. The test is real simple and like you Mike, I tried it with the hose still open just to see if the compressor would keep up pressure on the coach brakes. The 1/4 inch hose did not effect the coach brakes at all -- even when it was not connected and letting air out -- the pressure stayed up.
Dave and Karen -- '02 Ultimate Advantage 36 C, 350 HP Cummins, Allison 3060 Tranny
'07 Ford Escort, Brake Master 4160, '13 Ford Edge, InvisiBrake