The WFCO had wires to the DC panel and the battery has wires to the panel. Either can run the Dc circuits on that panel and the WFCO charges the batteries down the same wires that the battery has going to the panel. Those wires (neg mostly frame is common) are a two- way street.
The new converter just needs wires to the battery. It will supply the DC panel up the same wires the battery uses to the supply the panel. The only diff is if the battery is removed, to run the panel from the new converter you will need to join the wire lugs together that were on the battery posts with vice grip pliers or something.