On our 2016 Thor Axis 24.1 we went with two 6 volt LifeLine batteries turned on their sides. We had a piece of sheet metal added to protect the tops of the batteries from any debris under the chassis that may have been thrown up. You can search other posts but I do remember someone finding some 6 volt AGM batteries that would fit upright. They may have been BattleBorn brand. Either way, a good way to go.