Battery charge monitoring will take some getting used to. I can see now why it's important to let the battery stabilize, and can see that checking the voltage level while you are using it won't help much because it isn't accurate.
I turned off my shore power yesterday when the meter showed 13.5 volts. This morning, it read 12.7. I turned on the fan to medium and two interior light bulbs, and the meter dropped almost immediately to 12.4. An hour and 20 minutes later it read 12.1. After 4 1/2 hours, it read 12.0, which is supposedly down to 50% capacity. So, I turned off the fan and lights, and within 5 minutes, it was reading 12.4, or 80% capacity. Five minutes after that, it read 12.5, or 90% capacity.
I'll wait another hour and a half and log the voltage and resume power usage. I'll probably use power for 3 hour intervals and log the voltage before I turn it off and wait 3 hours and log it again and start the cycle over again tomorrow. This way, I'll get a rough idea of the amount of power I can use before needing to recharge. While at home or camping where there is shore power, this won't be necessary anyway as the converter/charger will be supplying all of the 12V power, not the battery.