The main living area audio in our 2015 Expedition 38K works the opposite of what you describe.  That is, to hear TV audio through the sound system, the mid (living area) TV must be turned on and tuned to the channel you want to watch.  Audio from the front TV is only available from the TV itself.  To hear it, you must turn up the front TV audio. I suspect that the designer assumed that living area occupants would normally want to watch the same channel on the front and mid-TVs.