Sounds like kaput batteries or the path that they are charged from. That is, dirty, loose connectors and cables. Start at the battery with the voltmeter on DC of course. Put the test leads on the center of the posts. If you are seeing extremely low voltages you need new ones. I believe your external charger was trying to tell you that. If that is the case, be sure to take pictures and count the wires connected to each location. Take the ground wires off and wire brush them shiny and clean. With new batteries in place start over measuring voltage at the battery. If you see 13.5 or so with the converter on your problems should be over.