if you are off grid camping a single 12 volt isn't going to last very long unless you have a generator or something to supplement. if you are always plugging into shore power then a single battery is probably fine.
regardless when you are parking an rv for an extended time disconnecting the battery is a good practice; install a disconnect switch at the battery if you don't already have one (they will be labeled; typically red in color and near the battery)
I wouldn't jump to replace a battery or buy a new pair; as you camp more and more you'll be finding tons of stuff to spend money on :)