Welcome to iRV2.
The only dumb question ; is the one you don't ask !
Particularly if not asking ends up costing you $$$.
Is the groaning , only on the first 3>6 brake applications of the day ?
Then not heard again till the coach has been sitting for several days?
Reason for asking ; my coach will have noise from the brakes , after sitting for extended period , and it comes from a slight build up of rust on the brake drums ( I live in an area with high humidity) and as soon as the rust wears off, the brakes a quiet for the rest of the trip.
Air pressure for the coach air system is provided by a compressor mounted on the engine and you can't hear it over the diesel operation.
Rapid cycling of the air dryer purge valve , could be an indication of servicing being required; most dryers have a 18>36 month service interval.
Are you aware that the air system in your coach may have , manual water drains on the tanks that should be used every 6 months to check for water accumulation.
There are several different air dryer systems in use on motor home chassis this document covers service on two of them ; just to give you an idea of the work required. If you find a different name on your dryer; and post it , I'm sure members can provide a service manual for it.