Put your voltmeter on the battery. Measure the voltage. Start the engine. Voltage should go up towards 14 volts. If it does, that circuit is good. Now you have to measure the output of the converter when plugged in right on the converter terminals. If it measure 14 volts, then measure the voltage at the battery. If it is not the same at the converter output, then there is a loose wire between the converter and the battery. It might be the positive feed, or the negative return. Use a jumper cable from the battery negative to the negative terminal on the converter. Did that fix it? It's the negative wire feed. Try the positive. Did that fix it, it's the positive.