It Depends Whoowee...
Most RV converters have the DC fuses in them as well. But that makes no sense in this case.
Look for a wacky design hookup FUBAR style that has each 12 volt accessory wired to both the converter and the plug (Y splits to two power sources) except for the water pump which only has a path to the CONverter DC connection.
After summer when you feel energetic, take a half dozen images of the entrance point of the external battery socket (the feeder wires running everywhere) and the converter wiring (behind the converter). It might be a rat's nest because the former owner made "some modifications".
No sweat. You have a simple to understand and explain system once images are put up on this forum :)