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The 12 volt
lighting in an Airstream works off the battery(ies) or the converter (converts AC (110 volt) power to DC (12 volt) power. This unit is the opposite of an inverter which takes DC to AC.
It sounds like the converter in your Airstream is not functioning. It could be caused by a number of things. The converter could have a tripped circuit breaker, it could be improperly wired, or it could be burned out or broken.