Hi Jess,
Your converter produces the battery charging that charges your battery when you are plugged in.
If the lights get brighter when you have the engine shut off, and you plug in to shore power your converter is working.
If the lights get brighter when you run the engine, then the 12 volt power from the alternator is working.
If the lights get brighter when the generator runs, the generator produces 120 volt power that then is sent to the converter, and that proves the converter is working, and the generator.
I hope this helps.