battery disconnect

ON mine the "OFF" position means "sorta off" or "almost off".

The emergency brake is still connected. The landing gear still works. The propane detector is still hot. There may be others that I just have not yet found. Disconnecting the ground at the battery is the only sure OFF.

Experimenting with interior lights is the way to check the positions.

A

It's just a switch that isolates the battery bank from the rest of the coach. They are sometimes wired in different ways. Sometimes wired to just completely isolate the batteries from everything. Sometimes wired so that the emergency brakes will still work, so you don't inadvertently go for a drive with no e-brake.

Turn the switch ON for normal trailer operation, including charging via the onboard converter. Turn OFF to eliminate all drain on the batteries, for example when storing the trailer. Turn OFF to safely remove or install batteries.

ON will be the normal position.
OFF will be the disconnected position.

Think of the switch as the equivalent of removing a cable from the battery post. In fact, I don't have one of these switches. I just remove a wing nut and remove one cable from the battery. The switch makes it more convenient, and arguably safer since it can't cause a spark near the battery, which can cause an explosion. But it really is just a convenience item.

On many units that switch is almost useless. It doesn't generally disconnect everything. As for which position, good question, on some units when in the ON position, it means it's disconnected, on others it means normal battery power is on.
Best to turn some internal lights on while running on battery power, I.E. not plugged in to AC or the TV. Then see what position the switch needs to be in to turn those internal lights off.