John gave good information. Here are some basics:
RV's have two nearly separate power systems. The 120V AC is clear. The 12V DC powers many things, including most lights, slide outs, motorized levelers / stabilizers, the furnace and ceiling fans. Other systems, including fridge and water heater require 12V DC for control circuits, regardless of primary power supply (Propane or 120V electric)
This 12V comes from the battery and / or the converter. The converter takes 120V AC and converts it to 12V DC to keep battery charged and supply 12V to the camper.
Since you have no 12V power, either the converter is not getting power (breaker tripped), the reverse polarity fuses are blown, or it is not working. Sounds like the converter from what you already checked.
Note also a fully charged battery puts out 12.6 V. A functioning converter puts out 13.5 V or higher. If your voltage at the battery is less than 13V, it is not receiving anything from the converter.