To check if the air dryer has failed, open each of the tanks DRAIN VALVES. If anything but clean, dry air comes out, the dryer has FAILED.
But, that may have caused your issue, but is unlikely to be the issue causing the leak.
Best advice, with proper jack stands in place, go under the coach with a spray bottle (kiddie bubbles and water are best). Spray every connection and valve. If it blows bubbles, fix it.
Yes, if the leak is large enough, as soon as you shut off the engine (i.e. air pressure at max/cut-out PSI) you may be able to hear the leak.