I would take this opportunity to clean up your battery installation
Firstly 8Ds are way to heavy for a mortal human to handle. So get rid of them.
A Group 31 battery will start any diesel
of 6 liters or less and weighs almost half of what an 8D weighs.
Then in the space that the other two 8Ds went, put in two pairs (four batteries) of golf cart batteries in Blue Sea or equivalent battery boxes wired in series/parallel. This will give you 440 amp hours of house capacity.
You can either wire the starting battery to one side of a 1,2,all swith and the house bank to the other side. Then you have to manipulate the switch when you hang out at anchor
for more than a day. Or you can wire the starting battery directly to the starter and the house battery to the alternator
and shore power charger
through the 1,2,all switch that will only be used for emergency
paralleling the two systems. Use a combiner or Echo Charger
to charge the starting battery. That is a better, set and forget system.
Flooded cell batteries do take a bit of work checking water
level and adding. In my experience in a moderate climate, I would check water
every 2-3 months but never had to add more than twice each year.
The starting battery can be an AGM, but why spend more money
. A flooded cell will be half or less.