Running the engine or generator or plugging into shore power should charge the batteries. If you have a positive terminal connected to a negative terminal between batteries you likely have two 6v batteries connected in series. If that's the case, do NOT hook a battery charger to one of the batteries. Hook the negative charger cable to the negative terminal that is NOT connected to the other battery and hook the positive cable to the OTHER battery positive cable that is not connected to the other battery.
The result will be charger on positive of battery one, negative of battery one connected to positive of battery two and charger on negative of battery two.
Traveling whenever possible with DW, 3 kids and "Tate" the mut dog. 1999 Minnie Winnie 3F430V