Best Way to Charge House Battery Bank

Please review #19 above. It's an isolator or a dc to dc converter. Think of a diode (isolator) as a one way electric check valve (one way, not the other). Same voltage only (less v drop for the diode). There are NO voltage dividers in this design.
Diodes are cheap and have a voltage drop, designs improve from here.
DC to DC converters are rather expensive (current = bux).
Please review #19 above again. I feel your thinking of a DC to DC converter.

Now, There is a dual relay system on Oceans and others. They engage to provide 24V to the mains starters, all else, the batteries are dual 12v banks.
What a cluster Frak.

Please send me a pic or schematic. We are all (me too) here to learn new stuff.

I think the "coffee cans" are actually serial/parallel relays, and the isolators you may be referring to are battery combiners like THESE. It gets confusing when manufacturers can't agree on a common product description, and it is worse in electrical products. Trying to find the serial/parallel relays on Google is a real challenge if you don't know the magic term. Nothing else will get you anywhere close (don't ask how I know )
Anyway, Capt J is right about battery combiners being the best solution, since multiple charging sources can be tied to a battery. The devices sense charging current availability and route it from the source to the battery.
I think avoiding a single point of failure with redundancy may be beyond the space limitations on the boat, though. That's mostly because of the size of cables involved. With big cables come big switches and circuit protection, not to mention minimum cable radius.
In terms of the 12 volt supply to electronics goes, a dedicated battery is a must, charged by an AC powered battery charger, with a battery combiner tie in to the gen battery. With that, if the battery charger dies, the gen battery/alternator will keep things going, if the gen has an on'board alternator. Some don't. A good sized alternator charging external needs on an alternator could also play hell with the RPM, therefore AC frequency. A switched battery combiner tied to one of the 12 volt batteries in a 24 volt bank could be used as an emergency charging source, too. Ultimately, I think that the issue of fail-safe engineering will result in too much space demand and then require monitoring or alarm systems. Without monitoring a chain of redundancy, you will still end up with a single-point failure scenario. Ultimately, a good quality, long set of jumper cables could get you home.

Edit forgot to include link for serial/parallel relays

http://www.texasindustrialelectric.com/relays_1119844.asp
The coffee cans are what I was meaning in #25 above.
It will not work for the OP as he is set up. His mains and chargers (& main alternators) are already 24V.
These relays are system fakers that drive 24V starters only and all else is 12Vdc.

There are no diodes or electronics inside. It just changes a 2 battery bank from parallel (12V) to serial (24V) while starting a main engine.
When the engine starts and is running it reverts back to an all 12V system.