Any 12v battery will do the job. The "specs" depend on whether you need a lot of amp-hour capacity or not and the physical space available. If you nearly always have shore power available, you don't need a lot of battery capacity (amp-hours), but if you want to "boondock" you probably want more. For physical size, the simple answer is to use the same as what you have, probably a Group 24 size. Standard BCI battery case sizes are numbers like 24, 27 (larger), 31, etc.
I've written an article on how to Choose an RV Battery, but it's on a different web site. Choosing an RV battery
2004 American Tradition; 2014 Buick LaCRosse
Homebase in the Ocala Nat'l Forest near Ocala, FL