The capacity of the battery bank is determined by your need for battery operation. If you always have shore power when stopped and don't need the inverter while driving, you can get by with a very small capacity bank. If you spend a lot of time boondocking, then the most capacity you can fit into the physical space is probably what you're looking for. Anything in between is up to you. Keep in mind that the bigger (in capacity) the bank the longer it will take to charge unless you change out your charger, which I would not advocate. The brand of battery doesn't matter. Some brands may last longer than others and they will likely cost a lot more as well. I just changed our my house bank and the batteries sold at Advance Auto were 20% cheaper than Interstates with the same capacity. You can also move away from wet cells to AGM, but you will pay a lot more. The great thing about AGMs is you don't have to worry about water levels any more. For me, checking the water twice a year did not offset the AGM price premium.
