I have a different system, but when my dash LED fails to light (with everything else working) it's because I don't have the coach's parking lights on. The electronics require that signal.
One other reason- According to the manufacturer, my braking system generally will not engage until the road speed is above 25 MPH or so (no connection to the speedometer, so it likely has to do with the lowest sensitivity setting for the inertia sensor). The dash LED therefore only lights when applying the brakes at or above that speed.
I know, different components- may not help you. Might, though.
2008 Holiday Rambler Admiral 30PDD (Ford F53 chassis)
2009 Honda Fit Sport