Hi albritton78,
Congratulations on your Big Country purchase and welcome to the Heartland Owners Forum. There's a lot of useful information here along with a great bunch of friendly and helpful people.
Our Landmark also has space for 2 batteries but came with one, which I believe is standard. If you want 2 batteries, I'd suggest asking the dealer to sell you a matching battery now. Mixing new and used batteries is said to not work well. Also, your rig may have come with a Group Size 24 deep cycle battery. If they fit, you'll probably do better with a pair of Group Size 27s. If you got a 24, maybe the dealer will trade up to a 27 since it's only a few days old.
You might find
this link helpful when you're ready to add the necessary wiring. The battery info starts a little past 1/2 way down the page.