On my GH, that is the switch that switches the entertainment system from indoor to outdoor. One way and the speakers outside play, another and the speakers inside play. At least on mine, you can tell from the drivers seat if the switch is set for "outside" because the front speakers in the cab play and the indoor speakers in the rv section do not.
I think I was unintentionally entertaining my neighbors while working inside my RV before I figured that out!
Tim in Canutillo