I've got a different converter in a different trailer. In mine the circuit breakers are on the 120V side and fuse on the 12V side. The rocker switch is for 12V from the converter on one side, off in the middle and 12V from the battery on the other. If you are not plugged in, you can test it with your interior lights
. On, off, off or the other way around. Plugged in you will get on, off, on.
on edit, reading your post above. It will probably be left = 12V from battery, middle = off, right = 12V from converter.