Please clarify whether "everything else" works on 12v means when not plugged into 120v.
If all 12v only works with 120v plugged in, the converter is doing the 12v work, and you have dead batts or an "open" between the batts and the DC panel.
If some 12v things work on battery but not all 12v things, then it is individual 12v fuses on the DC panel to check if those things are on the panel. Some 12v things like slides and jacks are direct to battery, not via panel, and can still work on battery even if the panel does not.
So we need the whole story to sort this out.