Hi Kenna,
Welcome to the Heartland Owners Forum. There's lots of useful information here along with a great bunch of friendly and helpful people.
Most people go with the one battery unless they're camping without shore power, or have a residential refrigerator, or have some other setup that uses an inverter to change 12V DC into 110V AC.
We got a 2nd battery because I wanted to run the refrigerator for 24 hours in the storage lot before bring the RV to the house to load up. The original battery would do that, but as it aged, sometimes it wouldn't have enough power left to reliably operate the hydraulics or start the generator. I replaced it with a matched set of group 27 12V batteries from Sam's Club.