First thought may not be the correct one, but maybe a place to start?
Looking at the drawings here:
Down on page 86, I see a radio switch that is a switch to control whether the radio gets power from start or coach battery, with the center being off. Maybe a check of it to assure it is in the correct position to power when you want on coach power?
I have had some experience with that switch on my RV and found that it was terribly confusing to sort out as the polarity seemed to change when in different positions but what I might think you will find is that the center two lugs of the six on the back of this switch will go to the radio and one set of outer lugs comes from start battery power and the other outer lugs from the coach power.
Some guessing on that info, so check/verify before trusting!!
Why we might want to do it, is not for me to say but the idea is that we can turn the radio on using either battery or none!
If you dig the switch out and check the lugs on back, that might be a start?
If you can find a code on the wires, like this example, it can then be "decoded", using the wire ID chart here:
Good luck on the bug hunt!