In the 1980s there were several add on 120/240 volt belt driven AC generators sold for pickups and service trucks, but a quick Google search does not bring up any currently sold.

I owned a Honeywell Redi-Line motor generator.
It was a “synchronous 12 volt motor” that was directly shaft coupled to a 1600 watt 120 volt 60 Hz AC single phase generator.
They had 1000, 800 and 400 watt models

It held 60 hz very steadily and the unit would not start running until it “sensed” a 120 volt AC load.

You hooked it directly to the battery terminals so that the battery amp-hours of capacity supplemented the alternator output.

Weighed about 40 lbs.

Worked without a hitch until stolen out of my truck tool box by a thief