The thing that it came with is a CONverter-charger.
An INverter produces 120v AC from a DC source (12v batteries in our case).
The old Univolt converters and even very recent Parallax converters have single-stage output, so they're not kind to batteries in the long run. Modern, affordable converters are available that provide 3- and 4-stage output to charge and maintain batteries more effectively and prolong battery life compared to the old-tech ones. Popular brands of reliable converters are Iota and Progressive Dynamics.
I'll move this thread to the electrical section where it belongs.
Il Carriaggio — 1975 Argosy 24 | Il Progetto — 1976 Argosy 28 Center Bath | WBCCI# 15566
He has all of the virtues I dislike and none of the vices I admire. — Sir Winston Churchill