You're mostly going to have to figure this out yourself, but recalling the original wiring on my 1990 CO 8', shore power came in to the upper shell via the outside 110V outlet and ran directly to the converter. Onboard 12V power went from the lower shell to the upper shell via an external cord, then to the converter. There were 2 circuits leaving the converter. One goes around the ceiling and feeds the lights (and potentially also vent fans), the other goes to the lower shell via an internal spiral extension cord and feeds the refrigerator. There's also a 110V circuit from the converter to the internal shore power outlet and the air conditioner junction box. So you probably have a short in the lighting circuit, probably from hot wire to ground, where ground is provided by a connection to the aluminum skin, which itself is grounded to the onboard battery via the external extension cord. Correcting that error, unless it occurs very near the converter, is likely to involve removing either the interior paneling or the exterior aluminum skin. Removing the aluminum skin is easier, just because of how these things are assembled.