Your second posts clarifies things. You stated in your first posts outlets lose power. This would indicate two independent systems failing simultaneously or at the minimum the circuit that feeds the outlet powering the converter. That can be removed from the equation since you now mention outlets do not lose power. What you’re experiencing is either a bad battery or a converter not putting out proper 12 volt current of 13.2-14.2 volts. Like someone mentioned, test the output of the converter and I would also load test the battery. If both of those pass then it lies between the 12 volt line in to the fuse panel or any number of the circuits that go dead.