smokinjoe3,

Pretty weird. Does the '72 have a printed circuit board instrument panel? I'm not real sure on this, but the fuel gauge might be giving you a clue. For all those dash indicator lights to come on, they had to get power, which might have found its way through the common ground. I don't know exactly how this would work, but if the fuel gauge was powered through its grounded side, it might have swung the needle to EMPTY.

If turning on the headlights initiated the problem, it could be a lamp that lost its ground that comes on with the headlamps. This could be a taillight or a dash light.

You might do this drill again at night, then walk around the rig and hunt for a dim exterior light. Check the dash for a dim or nonexistent light too. Something is feeding power back through the system in search of a ground...it's finding it, but lighting everything else up in the process. Good luck.

Stuart