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Hi! My name is ***** ***** I can assist you with this. I may need some additional information from you first in order to give a complete answer.
What is the make and model of your charging system? 13.9 VDC is about right on the charging voltage, but not if that is constant. It should taper down to barely 13 VDC when the batteries are fully charged. If they are dropping down that fast then either they are failing OR you have a drain on them. If you have a small digital meter (and it appears you do) then we can test for this. Most places that sell batteries can load test them for you. Batteries usually last about 3 years on most RV applications. Get back to me with this information and I can assist you further here. All follow ups are included in our fee. No time limit.
Just following up to see if perhaps you missed my earlier responses to your question? Easy to do considering the volume of e-mail that most of us get every day! If you need me to resend it please let know. Thanks!