Think with something that has been in service since 1972 and sounds like it had some less than ideal repairs and modifications done to the wiring probably smart to start over.
Known good can avoid a whole lot of "what now?" issues, especially with wiring and save time in the long run.
I do think a lot of trailer places run way to lite of wire for charging, been a lot of discussions on the forum about replacing that with something of a gauge suitable for RV charging. Some folks new to the whole thing do get caught short trying to run 3 way
fridge on 12 volt and arriving with a flat
battery.
That said 7 pin is the easiest and most standard way to accomplish that charging with the proper wire. I think that was the point being made (at least on my part)