Yes you do want your batteries to match. Either two 12v
batteries in parallel (+ to + and - to minus) or a pair of 6v batteries in series (- to + between batteries and then power out of the end.). Replacing the battery box can be a pain if you donít have one that can hold two.
The other option would be a single Lithium battery from Battleborn. They are designed to be a drop-in replacements and have about the capacity of two midlife AGM batteries because they can be discharged deeper. These have electronics built in to hopefully keep them from being damaged from mistreatment. They do, however, cost around $950.
The Victron battery monitor is about the best you can buy, thatís a fine choice.