An RV owner's best tool. A multimeter.
A multimeter is a very valuable tool. Usable meters start at about $50. The better the meter, the higher the cost. A multimeter will offer many answers
for anything electrical. A multimeter will indicate battery condition, if all the fuses are good and if the AC power is sufficient to use. When a battery indicates 12.0volts, the battery is discharged 75%. A full ready to work battery will show 12.6volts on a multimeter. To keep an RV battery always ready to work, get a small battery charger with a 20-50 amp output, get a 24 hour time clock. Hook up the charger to the battery, plug into the timer, set the timer to go on for a couple hours and off for about 3 hours. This method works on any battery. Some folks connect the converter, also known as a battery charger, and allow the timer to control the charging. A 12volt test lite is a handy tool for an RV too. Frank