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Hi my name is Robert...
When you are plugged into power the power converter should take the AC voltage and make the DC voltage so the battery can charge. Sometimes near the battery you might find an auto breaker or in-line fuse, below is a picture. Some are thermal and some have a tiny tab on them that pop out when they trip. As a general rule they like to stick them 18" from the battery and could be on the frame OR in a battery control box, the voltage on each side to chassis ground should be close.
The other issue is the converter, the converter will be part of the main fuse/breaker panel OR stand alone and plug into an outlet, the stand alone style normally will have two fuses on it. IF you know the make and model of the converter I might be able to help you more. Also if you can verify the battery voltage AT the batteries and you find you have good voltage please let me know as there could be another issue... Robert