[Bob Landry]

There are outlets wired to the output of the breaker that is marked converter, so if the vacuum, etc, work, the breaker is probably good. I assume that you are plugged into shore power, and if so, the converter overrides the battery for the operation of everything 12V and you don't even need a battery to run 12V components.
Disconnect the battery and you will probably still have dim lights, you have a bad converter. The reason the other lights get brighter when you turn one off is because the converter is running at reduced output and you are reducing the load on it, making the other lights brighter. Converters aren't rocket science.