Hooking up the two six volt in series to make a 2 case 12 volt battery is simple. Make sure the 6 volt batteries are identical in age and specifications. Hook up the positive terminal of one battery to positive lead, negative post of the other battery to ground, then a jumper from the empty negative post to the empty positive post of the other.
The only thing I didn't understand was, "24dc-12volt." You don't have 24 volt dc power in your coach, do you?
Also, you might audit your electrical usage, having to run the gennie every three hours seems like something's creating a big draw, unless you never let it run long enough to charge batteries back to full.