I have one water tank, on the centerline of the boat, under the v-berth right up in the bow.
I have three Firefly G31 batteries, one in the engine area under the stairs, and two as close to the centerline as possible in the lazarette. The PO had two large batteries in the laz, and a tiny starting battery in the engine area. I have one battery bank (not going to debate one vs two vs starting battery) and updated the entire system (covered in my articles under https://sailbits.com/tag/power-system-upgrade/) with appropriate wiring, fusing, and the works.
The G31s are lighter than what was there before, but I have considered moving one or more out of the laz to somewhere belowdecks, I've just not found enough space quite yet. The PO also had an air conditioner (!) installed under the galley sink, which would be more than enough space for the batteries, but I think the crew would keel haul me if I removed that, as it has become a favorite part of the boat while at the docks around Puget Sound and our hotter summers in the last few years.
There's no room behind the engine - that is a clamshell like door that is in the master berth, and again, that berth has no storage underneath any part of it for current battery types other than some of the newer lithium sizes I've seen, but that brings a whole new challenge to the equation.
Longer term I would like to move one or both of the batteries in the laz to the starboard side under the seating, but there are some unknowns in terms of where distribution would live, where the inverter would move to (if anywhere) and the like.
