On our Tuscany you cannot adjust the radio volume while the cameras are on. We travel with the rear camera on to see the Dinghy and it help to see people close behind you. You can turn camera on/off by holding down on the top right eject button that raises the entire screen (about 3 seconds). It would be nice to be able to adjust the volume while viewing.
Also, you do not need to disconnect the rear microphone. If you access the secondary audio screen you can turn the volume down to mute the rear microphone. We do not use it with engine running since you can't hear over the engine.