My ford truck built-in rear view camera always reduces the audio to almost nothing when I go to reverse and the rear view comes on. Not knowing anything about your system this may have have something to do with your reduced audio.
My rear view camera system has distance markers which beep. The closer you get to something the faster the beeps. This is the reason all other audio features get reduced so you can hear the beeps. ???
My Jensen radio in my OFF-ROAD POPUP works just fine - Nice and loud...
Just making comments???
Roy Ken