2 volt batteries are actually "cells" not batteries. A battery
being a group of connected cells. The basic "lead" chemistry produces about two volts, and one cell is one physical unit. In industrial uses like electric
fork lifts, it is normal to bolt up your own battery
from thesre industrial cells, and if one cell goes bad or weak, just replace the one cell. Given the heavy use those industrial batteries get--it makes very good dollars and sense for them to do it that way.
it certainly can work just as well for a boat, or anyone else, IF you are using that maount of power and IF you have the physical room to set up the installation
I think running the fridge directly from shore power
is a good idea, it simply guarantees that the fridge can't be "confusing" your charer, whatever charger you use. You might not *need* to do it that way but that is a simple fast reliable solution.
Given the cost of batteries, I'd start hooking up a good voltmeter and monitoring the actual charger output, and check with the maker to see how "smart" it is or isn't. It could just be broken (bad sensor, bad relay) and not dropping into float mode, with or without the fridge.
If your charger has the option of thermal sensors (for ambient or battery temperature) you might want to add that, ambient temperature is usually ignored but it does make a difference.
You certainly could use more capacity on the charger to charge faster--assuming your are using an AC genset to power the charger? Instead of direct DC from a genset?