You get no 12v when the battery bank is disconnected, but 9 volts with battery, so the batteries have been run down badly and first priority is to get them back up or be buying a new one (you only need one or none when on shore power and your converter is working.)
The fact you got power from battery to the DC fuse panel shows the "battery fuse" or DC breaker instead of a fuse, is ok and the positive path from DC panel to battery is ok, so the converter output to the DC panel should have been getting to the battery and not letting them get run down.
So that indicates the converter is not supplying DC to the DC panel and the battery lugs on that. Probably not since the water episode two weeks ago. Could be a loose converter wire into a lug on the panel front or back. (off chance)
You want to know if you have 12v DC above say 11v, will everything work? Check by using a battery that is charged instead of the near dead ones. You can run the engine and see if you get alternator charging, which will also run DC to the 12v system.
With the Parallax there were two 30amp fuses on the DC fuse panel, which were to protect the converter from reverse polarity hooking the battery wires on backwards. They would also blow for other events. With the PD, those are on the converter itself, which you checked. What about the two on the panel? (off - chance)
If everything works with a charged battery, then you don't need a whole new power centre. You may or may not need a converter, needs a "bench test" with 120v not from the input it has now (to eliminate that suspect)
If it is only the fridge that has "low DC" but the lights work, then there are things to check just for the fridge.
Also look for any DC breakers near the power centre -those little metal cover ones with a bolt/nut at either end. You can jumper those one at a time to see if any have failed to reset.
