We just Keep It Simple, and use the factory 1-2-all master switch. The house bank of the two big GC batteries is on position 1. Position 2 is the "emergency" battery choice.
What with a well insulated 12 volt refer box from the factory, and all-LED lighting, we can easily anchor out for two days (plus) and never exceed the useable capacity of the house bank. The house bank is rated at 260 AH, which effectively gives us about half that without harming the batteries. The longest time we have obtained out of this bank was 9 years before having to replace them; and they have always lasted at least 5 years.
Remember that the small diesels on boat like ours almost never need enough power to justify a separate "starting battery" like a much larger boat might need.
We do have a solid state multi stage shore power charger. Also, a Balmar Smartgauge to give an instant readout of % of capacity and voltage. I have maintained the redundancy of the factory expanded-scale analog volt meter as well. It's great to have (reasonably) fool proof methods of checking voltages.
And, in a nod to the requirements of insurance and the ABYC, we have two large-amp breakers adjacent to each battery pos. terminal, "just in case"....
