Hi Doug - Unless there's something non-standard about that switch, it's a true "disconnect". With that switch set to the disconnect side, the batteries are functionally disconnected and aren't charging. They won't charge from your vehicle while you drive and they won't charge at your house while it's plugged in if the batteries are disconnected.
Thinking of your batteries as living organisms, your goal is more of a measured approach than an all or nothing fully charged or fully discharged.
The decision to leave your battery plugged in depends a lot on your converter/charger. If you have a good multi-stage charger that you're confident in, then leaving your rig plugged in is great. Your batteries stay good and charged all the time and you're ready when it's time for. your next trip. You just have to be careful and check the water level every month and make sure that the charger isn't boiling the batteries dry.
I added solar panels a year ago and now my batteries are charging every day. Mine love the full charge state and they've never been healthier. I check the water levels regularly and things are fine. I haven't flipped the disconnect switch in over a year.
