Originally Posted by bubbles
The composite (typically the yellow is used) video is output from the theater system to the tv but the audio (red/white) is input to the theater system from the tv audio output ports to get theater system speakers for tv audio. In my case the theater system is selected for AUX AUDIO input. My GT378 is a 2012 and has the cheap RCA system as its theater system and thought the OP's 378 might have the identical set up. We're motorhomes, not toy haulers.
Interesting idea I need to check my coach. I think I have the yellow,white,red composite stuff, but it is audio IN on my TV and so you can just turn the volume up on the TV and not on the headunit if you prefer. Or do both, or just the headunit (the speakers inside, and/or outside).
If I reverse that and even use a red/white to mini-stereo adapter to plug in to the audio output (its a headphone port on my TV) then on the headunit, switch to the AUX audio in (with another converter), I probably dont need to fish a single cable!
TBD, as coach is in the shop. Maybe the OP may be in the same boat.