Well, if it's like Journey On, a 2005, you just lift the seat cushion and remove the fiberglass backing around the AC panel. Make sure that the AC is unplugged, cause it's 110 VAC at 30 amps. The 12 V outlet can be seen but it's a bear to remove it, even without the heater.
Journey Ons aft 12 VDC outlet didn't work for a long time. I checked the lead with a voltmeter and there was no power to the darn thing. Tracing backward, there is a small fuse strip just below the main DC panel, on the forward side (where all the wiring is.) The fuse that fed the aft 12 V outlet was missing., Put one in and now it works. You might check your circuit.
To show how tricky I am, I even removed the water pump form the heater cabinet without unbolting the water heater. Used Braille.
Boris