Liz, don't give up. It is easy. Take your multimeter over to the battery. Turn it on. Switch the dial or the controls to the 20 volt setting. Put the red contact on the positive terminal. Put the black one on the negative.
When your battery is on a charger, it will read (usually) around 13.7 or so.
When you disconnect the power, it will read around 12.9.
As soon as you run it with a load (like a light on or whatever) and then turn the light off, it'll read around 12.7.
If you test the battery (with no shore power) while you are running the lights and the furnace and the water pump, it'll read around 12. That does not mean you are harming the battery.
But if after turning off all of those devices and letting the battery rest, it reads 11.9, you have probably overtaxed the battery. If you do that all the time, the battery will wear out a lot faster than if you seldom let the resting charge drop below 12.1.
Now, if you want to get a little fancier, buy a small device called a hydrometer at your local auto parts store. It's like a big eye dropper, and they cost around ten bucks. This gadget measures the "specific gravity" of the water in the battery. Why? I am glad you asked.
The hydrometer tests the well-being of each of the six cells in your battery. If the specific gravity is high (around 1300, whatever that means), the cell is fine. When one or more cells drops below the readings for the other, you know you have a battery that is getting ready to fail.
So this test keeps you from getting stranded with a bad battery! Very useful, and not hard to do. Have an old rag handy while you are reading each cell -- the battery acid can drip a little. And wear rubber gloves.
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