The way I have been able to test the battery state of charge generally has been by first testing the amps used for a particular 12v appliance and then checking the battery discharge in volts. It's kind of like what should be expected and see if the testing makes sense. When we are dry camping in conserve mode, I will test the batteries once a day and see the voltage. The voltage reading will tell me the amps used, generally.
For me, it's just "close enough" to know my batteries are not discharging too far and it also tells me about how long I can dry camp.
This past weekend I attempted to figure out how much my batteries drained while using the furnace during the night but my voltage tests didn't make sense in the morning so I will try it again next year.
