That switch controls battery power TO the RV 12 volt circuits FROM the battery.
The red light will be on if its connected.
If you push it to OFF and the light goes out, the battery is disconnected.
If you push it to ON and the light does not come back on, its not on.
While plugged in, the red light may stay on all of the time, but your not getting charging to your battery unless the switch is set to ON.
You may have a problem with that switch circuit and relay.
With no shore or generator power and engine off, do your interior lights work ?