1. Gensets do NOT charge any batteries at all. They charge the appropriate battery bank from the battery systems charge system,
2. Some OEM's use a 120 volt battery minder/tender and connect to the chassis batteries to keep them charged when the genset is running and also when plugged into shore power
3. Some OEM's have a BIRD(Bi Directional system) that closes a relay to charge BOTH battery banks when on Genset or Shore Power.
4. The reason your chassis went dead is simple. It takes 12 volts constant supply to run the Genset and while it was running and since the chassis batteries are the batteries that start the genset, it slowly drained the batteries.
5. Odds are you do have a dual(BIRD) type system and is not working. Doug
