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The thing you want to do is start at the battery to see if it is putting out power. Unplug from shore power when doing this. If you have a multimeter or a 12 volt test light check from battery positive to negative to make sure the battery is putting out power. If so, check the circuit breaker on the side of the battery compartment, if there is a reset button push it, also check for 12 volts at those terminals. There should be two circuit breakers, one for the lift pump and the other for the rest of the twelve volt circuits. The one for the 12 volts should have a red wire and a black wire coming from the trailer compartment. The black wire should be from the converter and the red from the twelve volt circuits. The circuit breaker separates those from the battery. My guess that is where the problem lies, either the breaker is bad or needs reset. Some breakers don't have reset buttons and automatically reset. So, if you have twelve volts at one terminal and not at the other the breaker is bad or needs reset.
That's my first guess.