The lights are 12 volts DC operated by either the battery or the converter.
You could have two problems.
First
Disconnect from shore power, does the lights work? No, would indicate the circuit breaker or fuse near the battery is causing issue. It could be located under the front in small box(3X3)
Lights work. Could be problem with converter. Late models have reverse polarity fuses built into them. Locate the converter, look on end where the 12 volt wires connect. Fuses should be there, if equipped. Make and model of converter?