Originally Posted by Heckler56
That on/off switch is what makes the noise you would hear as you toggle the sliding door located switch. Those on/off switches have the solenoid or relay (I do not know the proper name) incorporated internally.
Hawk is right that they can get out of position by manually working them. Airstream did install on some units, OP’s vintage, a solenoid after production/delivery by the magnum inverter. Many have removed this as it draws phantom power.
I suspect you have a back switch or it is out of position.
The relay is not normally internally mounted, that faceplate/switch just provides the power to open and close the relay, the relay is mounted separately.
Typically motorhomes are wired so that you have a manual (rotary) switch in the main + line from the leisure batteries (this cuts the power from the leisure batteries), but then a battery isolator (so that leisure batteries can be charged from the starter battery/alternator). The output from the isolator is live as long as 1) the rotary switch is on (passing power from the leisure batteries) and/or 2) if the voltage of the starter battery is above the battery isolator threshold (ie the 12v
circuits can be live if the rotary switch is off, because the power can come from the starter battery). This means a second (relay) battery isolator is normally also fitted after the main battery isolator. This is energized from non-switched side (via a 5amp fuse) of the same relay typically along with propane/carbon monoxide alarm, and allows the faceplate to turn on the relay and so supply the DC circuits.
So, rotary switch just turns off leisure batteries, you can still have 12v
going to DC circuits unless the relay based isolator is closed and even then you still have 12v
on the non-switched side (to provide power to the relay but also to power alarms which are safety critical).
From your symptoms it is the relay that is not opening, the fact that you see the light going on and off means it is getting power and trying to switch the relay.
The switch is used in trailers but the wiring can be different.