Go with a modern converter like a Progressive Dynamics 9245. You can find many types on eBay.
It was common back in the day to fuse the common/ground side of the battery circuit. But it seems odd, if this were the common/ground side. Why would it come in next to the +12 volt from the trailer and car battery. Can you get a pic of this bracket? It may in fact be a 30 to 50 amp fuse holder.
Your question of the capacity of the old Univolt. In my experience they were 30 amp. That's the size of the fuse going to the trailer or house battery.
In most trailers the black wire is the + side of the 12 volt
If you know how to use a VOM, it should be easy to identify. By continuity or reading voltage.