I know thus is a stupid question but are you sure all the wires are connected in the right place? If the batteries are good and will pass a load test then it sounds like one of them is connected to a hard ground instead of the starter motor. Or a power wire has rubbed into the frame around a corner or something. I would disconnect and isolate all wires and meggar each one to 250 VDC. If there is a short to ground you'll find it.