First make sure you have your DC meter on the converter's terminals to check it, and not just go with the wires that go on the battery. There are possible "opens" between converter and battery like a fuse or breaker or a wire come out of a set screw terminal on the DC fuse panel.
Ok so you hardwired the PD to the same 120v as the Parallax had, which would have been just the two wires, black (hot) to the breaker and white (neutral)to the N buss. There was no green neg ground.
The Parallax converter (lower portion) in its metal case touching the big metal case for the whole power centre got its ground via the big metal case which is grounded to the RV frame.
The green wire from the PD I think should be grounded to that metal case somehow, but somebody else here should confirm.
You would still have the same 120v breaker for the input to the PD, which you can check. You can use your multi-meter on 120 to see if you have 120 on the converter side of the breaker where you wired it in. Then see if there is DC out. You should suspect your wiring job to see if it is still good.
I think your "Parallax" is really a Magnetek 6300 from that long ago, and with the 6300 gone, you would have needed a DC fuse panel that did not split for dirty and clean DC like the 6300s did. If you did not do that, you still can. (You can jumper the two pos red and blue on the old 6300's panel to fix that) The 7300 was all clean, so no problem. That is still the case about the fuse panel even if your do have to replace the PD.
