Here's a VOLTAGE CHART that I carry around with me to refer to when asked questions about using VOLTAGE as a guide to determine the state of charge of one's battery system...
Keep in mind that VOLTAGE can only be used in this manner if the battery system has been at 'rest' (no charging or discharging) for some time to allow the batteries to stabilize, and with recent charging, to allow the battery plate 'surface charge' to thoroughly penetrate all the porous, nooks and crannies of the plates... It also allows the battery fluid (electrolyte) to be thoroughly mixed again as it becomes 'more acidic' during the charging process and migrates evenly around the plates - there's lots of 'chemistry' going on inside those 'electron containers' we call batteries!
I like to let the batteries 'rest' a few hours, or over night at best...
The voltages shown below would be at the battery terminals for the most accurate readings - this being impractical for most of us, you could also read at the battery cables where they attach to your fuse panel to get a usable reading...if you measure at a 12 volt
receptacle, the readings you get may be slightly 'less' due to increased resistance in the size and length of the wires from the batteries all the way to your measuring point...
16.00 - Equalizing - Flooded Cell Types
14.36 - Gassing
13.46 - Float
12.78 - 100% - Flooded, AGM or Gel Types
12.48 - 75%
12.16 - 50%
11.96 - 25%
As you can see, there is very little difference in voltage between 100% and 50%, so an accurate digital voltmeter is required to use this method of determining state of charge of your system...
I've installed a 'Volt Minder' that I got from BestConverter.com, about $25 as I remember...The digital meter on the 'Minder' has large numerals (xx.xx) and is east to read...I installed it with the leads attached to the battery cables at the fuse panel, and mounted the meter nearby...the Volt Minder has a low voltage 'alarm' that you can set yourself to remind you that your battery system needs charging - neat gadget! The VM does 'draw' a very small amount of current while hooked up, so you may wish to add a switch, or unplug it when your RV is in storage...