My coach has a Jensen radio in the coach that feeds the inside and outside speakers. On my unit I have a set of 3 buttons. Marked "A", "B" and "C". The "A" button operates the interior front speakers. The "B" button operates the bedroom speakers and the "C" operates the outside speakers. Those 3 buttons only turn them on or off. The Jensen system volume switch controls how much sound goes through it. This may or may not help you with your system. You can try looking it up on YouTube for help.
We tried a sound bar but the volume was not sufficient. We considered a better sound bar but then instead decided to run a 15 foot male-male line from the TV RCA audio out jack to the dash radio aux 3.5mm input jack. This worked well for us.
I do it with a 1/8 cable from the headphone jack on the TV to the aux in on the dash radio.
That's what I do as well. A more elegant solution would be to run the cable permanently from the TV into the back of the stereo instead of plugging into an aux input in the dash, but routing that cable behind the interior panels would be a pain.