Well, I only have 2 batteries... And they are on a switch so they tend to run isolated the vast majority of the time. I bought a couple of these 12v LED voltage gauges for under $5 each off ebay. My batteries are under the observer seat, so I mounted the gauges in the base of the seat. Cheap, simple, effective.
Just thought I would throw out a related solution for those of us with a lot simpler systems that are down for a cost effective solution.