Alternators won't do it?

The (very large) alternator will power the onboard electrical systems and charge the chassis/equipment batteries while the engine is running.

When sitting in a truck bay, there’s a shore line providing mains power to keep everything charged — we’re generally talking a 10-16A draw.

That’s easily met with a small generator, as opposed to the insane power draw a full EV battery would require.

The fire apparatus I’ve worked on have a 120V inlet called a shoreline to keep equipment (MDT, radios, cardiac monitor, portable suction, and Lucas battery chargers, interior lighting, etc) operational at station without draining the battery.