I'm going through this exact same planning process myself right now, so I have many more questions than answers. But if it were me, I don't know if I'd dig the center of gravity with a tank like that. I think I"d want something flatter shorter and wider. My original plan was to balance the batteries w/ water. But then you're forced to locate your grey/waste water in pretty much the same location lest your balance be off. And you're forced to keep the same volume on hand for the same reasons. Yet to implement, but my new plan involves two water tanks on either side (balancing each other), with the battery bank either centered, split (a bit problematic), or balanced by some other static weight source. Course that plan'll probably change 20 times before we put the rubber to the road.