If your rig is four or five years old you might need a new battery. If you do any dry camping especially if you are dry camping in the winter and are running your furnace you will want at least two batteries. One battery will not get you through a cold night. If you don't do any dry camping then you only need one battery.
Battery boxes should be vented unless you are using sealed batteries.
I dry camp so I have four batteries in a vented box that sits on the tongue of my trailer.