My two battery monitor meters jump around some during the one day/night run off the battery based on what is on at the time. Since both battery battery banks are selected both meters read the same DC VOLTS. With my BLUE SEA Battery switch setup however I can isolate the two battery banks or even just look at any battery mounted on the trailer tongue. I usually switch to either one bank or the other on occasion to see how each battery bank is doing. This lets the switched out battery bank go to its rest voltage pretty quick.
Like said above this only gives me approximately state of charge and I have been doing this since 2009 on these GP24 Interstate 12VDC batteries and all still going good.
This is how my GP24 12VDC batteries are wired up.

Roy Ken