Disconnect from shore power and remove a battery lead from the battery(ies). Your choice, they both work to kill the circuit. I like "+".
Open the door of the power center. Remove the 2 screws that keep the front in place. Lift and remove front. On the right side/half is all the 12v circuits. Note the large 6/8 gauge wire coming from behind and going to the "+ Batt" terminal of the fuse panel. (it's the one attaching at the bottom of the panel) Do not confuse it with the wire coming from the converter.
Now, remove the 6-8 screws holding the entire power distribution box in place. Pull it forward/out. Disconnect battery feed wire. Figure out how much run you have in the wire coming from the battery. Drill a hole and mount the switch where you want it within that range, or add to the length if needed. Run a new piece of wire from the other side of the switch back to the "+ Batt" connection on the distribution board. Verify switch operation. Reinstall box and cover.
Enjoy celebratory beverage.