Several things are 12VDC and gets its DC power from the converter/charger unit when plugged into shore power power. Also the Battery is connected to the same 12VDC Power Distribution panel so your ceiling lights should have stayed on but apparently your battery must not be connected or dead or switched off...
The other very important thing will be your on-board batteries will not be charged when plugged into shore power without the working converter/charger unit.
This is s simplified RV 30AMP Wiring Configuration diagram showing the 120VAC and 12VDC controlled items.

As you notice all the essential appliances are wired to 12VDC so that the camping off the power grid concept can be achieved. Only a few non essential appliances are powered only by 120VAC.
Roy Ken