Start by telling us what type of batteries that you have and how old they are and what kind of charger you use(how many amp output)? When was the last time the terminals were cleaned of corrosion? When you returned home on the 30th did you fully recharge the batteries? If so that much self discharge in 7 days is too much. If you did not fully recharge just drove home, that could be a normal reading. I assume your battery disconnect switch is on? So take some readings at your 12v panel and see if you read 12.3 v(same as battery unplugged) and let us know.