First off, you got the right thing, but it's generally called a CONverter. An INverter is for taking a 12v
DC input and producing a 120v AC output, the CONverter does pretty much the opposite, with the added design criteria to provide for charging 12v
In my '75 and '76 Argosy trailers, the fuse block was an integral part of the original converter (and the '76 converter looked pretty much like yours, I never saw the '75 as it was replaced by the previous owner.) If that's the case with yours, you'll want to buy a 12v
fuse block to go along with the new converter, but if you already have an external fuse block it's an easy swap.
There may be 1 extra output wire on the old one, if your trailer has the optional-for-Argosy central control panel. This lights up a light saying you have "shore power" when you plug in, so it's no great loss IMHO.
The basic layout is that the + and - leads from the converter attach to the corresponding lugs on the fuse block, as the + and - leads from the battery attach to the fuse block. There should be big fuses near the battery on at least the + cable to protect against shorting that main cable to ground.
The various color-coded wires that supply 12v DC to systems in the trailer should attach on the output size of the fuse block, with an appropriately-sized fuse placed in the fuse holder.
Hunt around on the site, there are some good photos of updated 12v setups on 70s trailers.