dual power

Hi guys
Need recomendations on a new project...
Currently everything electrical is run off my start batteries. Inverter, lights, fridge, accerssories etc etc. I'm wanting to move all the domestic and non essential load off the start batteries onto a domestic power bank. Trouble is in NZ (Kiwi here :) our patrols are 24v, but the vast majority of everything electrical is 12v. Except my driving lights and instrument cluster. I have to run a 12v reducer for my stereo and everything else.
Ive been getting dangerously close to dead batteries in a 24v automatic diesel, and over here those are hard to jump start unless you have another patrol or big rig around to help.

So my idea was to set up a domestic 24v system along side the starting batteries, with a isolator between the two systems for charging purposes, and then a decent reducer for the accessories. This way I only have one voltage I need to charge.
The other option is to set up a 12v power bank for the accessories, which would be easier, I just don't know how I would charge it.

nothing i can do about the 24v factory system though cause the auto runs 24v through the computer and the solenoids.