what did this trailer have? was there an old converter that died, and you want to replace/upgrade it ? I'm afraid I don't know anything about Trailmite, nor what sort of power converters were used circa 1972...
I'd likely be looking at a wfco or pd4600, maybe 30-45 amps, assuming you have a group 27 or 29 sized marine/rv
battery. theese come either as just a converter that you slide into an existing power center (assuming you have one thats apprporiately sized), or they come as a complete power center, to which you'd wire your AC supply cord, all your AC sockets and circuits, and your
battery and all your DC circuits.
example:
https://www.progressivedyn.com/rv/po...charge-wizard/
I do note most all of these are designed for a 30A AC input circuit. I also note that both of my 30A trailers work just fine on a 15A extension cord in the driveway at home as long as I'm not using
air conditioning.