I use a standard DC VOLTMETER setup for my battery monitor panel that I can watch from the inside of my trailer.
This is not absolute but is close enough without getting into all the inner workings of things...
I use this simple CHART to determine the % of charge for my battery bank when camping off the power grid.
When I read around 12.0VDC I know my battery bank is somewhere around 50% charge state. This is where I either quit using the batteries or start re-charging using my smart mode on-board converter/charger unit direct connected to my 2KW Honda generator using a RV30A to 15A Adapter.
I plan out what is being used when camping off the power grid so that my batteries make it through the one day/night run off the batteries and end up with around the 12.0VDC reading at 8Am the next morning.
Here on the east side of the USA this usually fits in with most of the campgrounds that have generator run time restrictions in place and I can re-charge my batteries back up to their 90% charge state in as quick as three hours so we can do all of this all over again for the next day/night run off the batteries.
Works pretty successful for us when camping off the power grid...
Roy Ken