Another possible spot may be your ignition switch. At that point your charging and other load(lighting,etc.) circuits are close. At least according to the C-5/G-5 wiring diagram. You said you shut off your lights and she started,right? Hummm........
Or you may also have some back feeding or load sharing going on. That tank looks retro fitted, how is the wiring harness underneath , was it moved around to accommodate the tank?