Your rig likely came with one group 24 12 volt battery - if you choose to add a second you would need to set it up parallel (as opposed to series). Easy Google to see how to do that.
As others have indicated - the std battery status lights that come with RV's are pretty much worthless - have your current battery tested at any auto tire/parts house (it's free).
If your going to change batteries you have the option of switching to six volt deep cycle batteries - purchased in pairs and setup in series to obtain 12 volts. Some consider these superior to 12 volt batteries. The six volt batteries are taller than 12 volts and won't fit in some Class C's (they don't fit in mine). My rig has 2 Class 24 batteries which I purchase from Costco.
Hope this helps.