Don't have a generator and PO had installed 4 group 27 batteries in the port cockpit locker; two in the original FRP box and two in additional plastic boxes. I wanted to get rid of the lead acid batteries and go to Trojan AGM group 31 which are about 1 1/2" longer. Took out the FRP box, cut it down the middle, added enough to hold the larger batteries, fiberglassed it back together and reinstalled it with 5200. The plastic boxes fit fine. The PO had the batteries wired with 3 in the house group and 1 in the engine start. Didn't think that was right but ProMariner told me it was fine and the battery charger would do OK. After that I installed a larger more modern 40amp ProMariner 12-40 P. Someday, I would like to use the 4 Group 31 as a house bank and then add a smaller Group 24 for engine starting on it's own charger.
I like Seadadler's solution because that compartment is difficult to reach for storage, so a good place for sealed vrla batteries; incredible you used the same cables. Don't know if it would unbalance the boat without a generator; although mine is a tiny bit to port. The under berth would be great also but don't know if there is enough room for 4 batteries.