Interesting. Perhaps the previous owner reorganized the furniture below the front window to a custom layout that didn't leave space for the batteries and converter? I think there would originally only be a flat plate (or nothing) where the box is mounted in your A-frame, with 2 batteries inside battery boxes that went through the skin where you can see the rectangular patches. The Univolt would have been inside the trailer between those boxes.
It doesn't seem like a good place for the vent on the bottom of that box, where water could splash up in there while driving in the rain, right before you plug in at a campsite. Is there at least a cover you've removed to show us the innards of that box from the underside?
If you remove Univolt and there isn't already a more-modern converter installed elsewhere, you'll have some decisions to make. You could use sealed batteries on the interior, rebuild the battery boxes or replace the box built into the A-frame with something like the modern battery boxes that go there on newer trailers and install a modern converter-charger just inside the trailer to keep the charging wires short.
The breakaway switch should have 12V
coming in on one wire, and the other wire should connect to the brake wire somewhere along its journey from the umbilical to the brakes. In better builds there's a junction block somewhere accessible for the umbilical wires to be joined to the trailer side of the various connections, but my 24' Argosy (for example) just has the various wires joined together just inside the belly pan.