William,
Considering you have a 2003 Contessa with a Cummins engine, perhaps you do not have an Echo Charger. Yes, you can use an automotive type of charger to charge your chassis batteries. They may not take a charge because they are too far drained, but still try. Then see if you can start the generator to charge up both set of batteries using the onboard inverter/charger. Hopefully, your engine will start once the batteries are fully charged.
Good luck,