If you hear the relay click, you are getting control power to it.
You need to locate the relay, then check each side with a volt meter. They may not read the same or should not read the same.
There should be chassis battery voltage on one side and coach battery power on the other. This is on the large terminals. If no power is seen on one of the terminals there is an open conductor or bad connection between it and the associated battery.
Then have a helper hold the switch in to operate the relay. The relay large terminals should read the same. If not the relay is bad.