Terry I replaced my WFCO with the PD 4655. My converter is about 25' from the batteries and had the same #6 wire. The wire runs through the floor joists and I couldn't get to it, so I added a #4 wire running underneath the floor inside some split loom and zip tied it to the black pipe propane line. So I have two (+) lines running from the converter to the batteries, one #6 and one #4, giving me a combined capacity that is slightly greater than a #2 wire. The two wires start and end at the same points, just take separate paths. The (-) path is the frame, so no capacity issues there.
You definitely need to either upgrade the wire or shorten the path like RoyB did to get the performance out of the PD converter. In my case RoyB's solution presented more wiring issues that it solved. Whatever you do is going to depend on your floor plan and what fits best.