How Do I Charge, and Maintain 2 New 6 Volt Batteries???

I bet they will work great. After a few runs down to 50% and then good recharges, their capacity will actually increase. And you have the genset so I think you are in good shape.

The truck recharge is a great option too, but it will probably burn more gas than the genset.

If he has a portable charger he can charge his 24 while you are charging your batteries too.

Do you have any way to measure amps coming out of your converter, or total amps into the battery?

Jim