I have a 2018 Fleetwood Bounder 36H that has a rocker switch behind the sliding door above the passenger seat don't have any idea what it does? Click For Full-Size Image. I also have a HDMI cab...
Well two good guesses up there 1: OTA antenna amplifier.. If you do not have either a matrix switch (Box of many buttons) with a POWER button. or a wall plate with teh 12 volt outlet, antenna connection, switch and light. or a Sensar Pro. that's a good chance.
2: Satellite antenna (rooftop dome) you will have a Dome up there. The lack of "Winegard" on the switch suggests a dome.