Deep Cycle Battery Question

You didn't say what kind of batteries. The above assume FLA or flooded lead acid. These will need distilled water from time to time. With the caps removed they should be filled to the bottom of the split ring, looks like a tube with a slot going about an inch into the battery. Fill to the bottom of this. AGM's, absorbed glass mats and Gel cells don't require adding water. Look up the battery manufacturers charging specs for your batteries and make sure the charger you use can meet those specs. Never mix old and new batteries; say 6 months to a year between manufacturing dates although no more then a month apart is better.