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

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.