They replaced the charger that was overcharging, making the dual
lead-acid batteries release hydrogen. So far, the CO sensor is not
registering any H2 with the new charger.

Here's the problem: The guy who installed the new charger said that as
long as there is no H2 registering, keep the charger plugged-in, even
after the green light is lit (full charge). However, the
manufacturer's instructions say: "At green light, keep charging for a
couple of hours for complete charging. When charging finished (Green
LED), disconnect from mains and remove clips from battery. Charging
can continue in 'float stage' without harming the battery."

What does all this mean? Do I disconnect, or not?

Thanks,
Jack