Given that you forget to check your batteries.... go with AGM batteries that are sealed and therefore need little maintenance, other than cleaning the terminals occasionally.
Interstate does not make the best deep cycle batteries. If a salesman doesn't know what you mean by amp-hours, that's a sign not to buy one. Lifeline is the recommended brand here in AGM.
Personally I would go with 2 batteries that I could move myself, instead of one huge one. Either 2X 12V in parallel or 2X 6V in series, crunch the numbers to see which will fit in your space and give you the most amp-hours.
