Oh no, another science lecture! Before y'all doze off or start manufacturing spitballs, let me just point out that this will be a short, simple, and non-technical explanation about what's going on with your batteries and how to tell the true state of charge of the batteries in your RV. So sit up straight and pay attention for a few minutes. There will be no equations, very few numbers, and a minimum of technical terms. Come on, you can do this.
The reason there's not a simple relationship between battery voltage (what a voltmeter reads, or how many of those four idiot lights are illuminated on your display) and state of charge (what the actual storage status of your battery is, 100% being fully charged and 0% being flat dead) is that your RV system is constantly doing different stuff – electricity is coming and going all the time. There is a simple relationship between the two for a battery that's disconnected from all charge sources and loads and has been sitting an hour or so, but you don't want to do that every time you check the system. It's a good place to start, though. This is called the “open cell” or “resting” voltage of the battery. Resting fully charged 12 volt batteries are around 12.8-12.9 volts, and flat dead ones are at 12.0 volts, so 12.4 volts on a resting battery means it's about 50% charged.
In general, charging inputs raise the actual voltage on the battery above its resting voltage, and loads (drains on the battery) lower it below its resting voltage. Crank up your Onan or engine generator, or plug into shore power, and your voltage goes up to 14.0 – 14.5 volts, even when your battery is seriously depleted. Turn on a big load like the microwave (if you have a big inverter that will power it), and voltage dives down into the 11.5-11.8 volt range, even on fully charged batteries. Again in general, the bigger the charging current or load, the further above or below your resting voltage your system will be. So looking at the voltage doesn't give you an accurate reading on how charged your battery is – you have to adjust for loads and charging sources.
The foolproof way to accurately determine state of charge is to use a hydrometer – a simple device that measures the density of your battery electrolyte. Fully charged batteries have lots of sulfuric acid in the electrolyte, depleted ones don't – the acid changes into lead sulfate on the battery plates as the battery discharges. Since sulfuric acid is heavier than water, fully charged batteries have electrolyte specific gravity in the 1.255-1.275 range, 50% depleted batteries read 1.175-1.195, and dead batteries read 1.095-1.115. A hydrometer can be purchased at any auto parts store for less than $10. In addition to the specific gravity numbers, they'll have color-coded ranges that say good battery, so-so battery, or dead battery.
Now, to use the hydrometer you have to pull out your battery tray, open one of the caps, and suck up some electrolyte into the hydrometer, so you won't be doing this often, or on the road. If you have AGM batteries like the newer Roadtreks do, you won't be doing this, period – you can't get to the electrolyte at all because there are no caps. For us traditionalists, though, you can use the hydrometer to calibrate your voltage display under known load conditions, and you'll know what voltage reading goes with which state of charge.
What do I mean by “known load conditions”? Well, in my Roadtrek, for instance, the TV is always on, and we usually have a fan going, and one or two inside lights are turned on. I charge up my batteries, verify that they're charged by making sure the hydrometer reading is in the 1.26 neighborhood, and look at my voltage display when this stuff is running. Three idiot lights and 12.5 volts on the meter. Under this load, that's fully charged. Now I leave the stuff on for a few hours, go back with the hydrometer, and see that specific gravity is down to 1.18 – half discharged. My voltage meter says 11.7 and I have one idiot light on. Now I know that, under this load of TV plus fan and lights, this is the point that I need to recharge my batteries – don't run your batteries below 50% unless you're in an emergency situation. Now I can put the hydrometer away – I have a voltage number, 11.7, that corresponds to 50% discharged and time to run the generator. Easy.
People have different numbers or different types of batteries and different typical loads, so 11.7 volts on your system may mean more or less than 50% discharged, but the principle is the same – there's a particular voltage that corresponds to different states of charge for your batteries under load. For those of you still awake, here's a discussion of this by Surrette, the fanciest battery manufacturer around. The voltage figure their table has for 50% discharge is 11.58 volts, so that's probably using a larger load than mine (they talk about turning all your loads on, which is crazy in my RT), but test your own system and use a load that's easy to establish – for me it's VERY easy because the TV is on as soon as my wife wakes up, and stays on until she dozes off and I turn it off. With this load on your batteries, there's a foolproof battery voltage that corresponds to 50% discharge. Stay above that voltage, and you and your batteries will live long, healthy lives.