Hi cabells,
Welcome to the Heartland Owners Forum and Family. We have a great bunch of people here with lots of information and all willing to share their knowledge if needed.
If your batteries are hooked up correctly, positive to a negative and then just like your 12v was hooked up with the positive and negative cables,,, you would have no problems... You just have a 12v battery. A BIG 12v battery. Your not going to 6v,,, your still at 12v, just more capacity to run lights and stuff.
Enjoy the forum and I am moving this out of the ATF thread.
Jim M