[rickst29]

12v appliances include the radio (if equipped), the fridge (on 12V power), and the gas detector (which should be lit when the bathroom back wall is UP). And of course, the 12V "cigarette socket" outlets are 12V.

I ask because the front shell lighting and rear shell lighting go through different fuses, implying that maybe ALL 12v is dead.

If plugged in: your converter has gone bad, AND your batteries have lost too much charge.

If not plugged in: then only your batteries are dead. (but your first post says that 120V outlets and appliances are working.)
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To check via the fridge, switch refrigerator mode to 12V and raise the "coldness" dial to maximum. Open the lower fridge vent cover, and feel whether heat is being generated (by the 12V heater) on the insulated right-side "stack" on tubing at the top (but be careful checking the tubing, it gets VERY hot when running correctly).

To check via Radio: Just turn it on, play some music.

To check via 12V socket- if you don't have something which fits, then put one voltmeter probe to the button in the middle of the bottom, and the other probe to the metal side conductor (partway up). You should read a difference somewhat greater than 12.0 Volts (positive or negative).