The Charge Only/Auto-Invert switch you refer to, is disabled unless you or the previous owner had unplugged a headphone style jack from the back of it. Read through the post I provided here as it explains in detail, the purpose of it all. If you are still confused afterward, just reply again with your question and I will try my best to answer it with better clarity.
CLICK HERE-------> http://phoenixusarv.com/community/index.php?topic=73.0
Quick summary for the 2007s (any many other years too)......The factory wired the RV such that all 12V opperations including power to the inverter is controlled by the switch at the entry door. If dry camping without shore power, killing 12V power at the entry door, kills the fridge on propane, spoiling all food. The solution is to leave 12V on, but this keeps the inverter powered up all the time using 12 amps for nothing, draining the batteries unneccessarily.
Unplugging the head-phone style jack at the Trpp-Lite control panel, allows 12V opperations to stay on in the RV (importance to the fridge), but then allows you to switch the Tripp-Lite control panel to Charge Only which shuts it down, saving a lot of 12v battery reserves.
We would leave camp in our tow vehicle, leaving the motor home at a primitive camp site for the entire day. Gone all day, the inverter was running for nothing, just so the fridge could run. With the jack unplugged, we can control the inverter independantly of the rest of the RV, conserving lots of battery power.
The flip side to this is, you can forget to turn on the inverter when plugged into 110V shore power. The factory setup is geared toward RV park travelers, not people who do a lot of primitive camping.
I might have a little of this wrong, but I think my input is accurate.