I am not really sure that I understand your question. The lights and other 12 Volt DC appliances in your RV always run on 12 Volt DC.
When your RV is not plugged into electric the RV's house batteries provide the 12 volt power. When you are plugged into electric your RV's converter takes over the job of supplying the 12 Volt power to your lights, water pump, furnace blower motor, etc.
A sure sign of having a converter problem is if the lights in your RV start to go dim when your RV is plugged into electricity.
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