First let me reassure you that there is nothing wrong with your travel trailer. The reason you are not getting 120 volt electricity in your RV when running on battery alone is because you need a 12 Volt To 120 Volt Power Inverter to transform 12 volt DC current to 120 volt AC current.
Power inverters come in all shapes and sizes. Some smaller inverters can be simply plugged into a cigarette lighter socket; others have to be hard wired to the house batteries on the RV. Power inverters are rated by wattage. Depending on what types of appliances you want to run will determine what wattage power inverter you need.
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