The Power Converter supplies 12V DC to the fuse box and to the batteries to keep them charged. Sounds like yours is not working and the batteries are run down. Before taking things apart, check the circuit breakers. One of them supplies 120V AC to the Power Converter. Flip it OFF and ON to make sure it's not tripped.
The Converter is behind the rear wall of the pass through basement storage. Next thing to check is to see if it's plugged into the outlet back there. If not, when you plug it in, tie it down so it doesn't come unplugged again.
Next thing would be to check the auto style fuses on the Converter. If batteries were recently disconnected/reconnected, momentarily applying the cables in the wrong polarity will blow the fuses.
If everything looks good, you might have a broken Power Converter. A temporary workaround would be to buy a battery charger from Walmart (get the biggest you can find), and connect it directly to the battery terminals. That'll get your batteries recharged so that slides and landing gear will work, and your 12V systems will work, including the thermostats.