Volt meters suck,.....unless you are experienced at using them,....in your own RV, and then only at a given time w/ a given load. You can have a resting battery reading good-green-12.4-5 volts, but as soon as the furnace kicks on, will be in the red, and should have been recharged the day before already.
If you are inexperienced at using voltmeters, the best battery state of charge meter you can use, and rely on, is watching an incandescent light while someone turns on a faucet to fire up the water pump. Normally, if you see any lite dimming, you are on borrowed battery time, and know your battery will not make it till morning, especially if using the furnace.
OTOH, if there is no lite dimming, you will know that you are good for all nite an maybe the next day,...depending on your 12v usage.
Just stressing the fact that you must know the load on the battery before looking at a voltmeter of any kind.
