Single big battery v/s 2 batteries in parallel

Too late now, but it is better to not take the single 12v battery the Dealer issues with your new Rv, but instead get a money credit for that, which you can use to help pay for the batteries you really want/need. (like two 6s or whatever you prefer)

As it is, you can indeed get a second one while the first is still fairly new and it will be fine to run them together.

Having more AH is great, but it also means you need to recharge more AH back in. Which is worth another dozen threads! :)