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According to your posting and question the batteries are over 4 years old. Most won't go over 3 years. Start by having them load tested and clean the cables, including the leads to the chassis ground. A dirty ground can cause low voltage as well. Then we can look to the charging side once that part is resolved. Take a photo of the cables and hookups so you get them back the same way. All follow ups are included in our fee. I will make a phone chat option available for you in case you feel that would be more helpful here. I can supply a Service Manual in case you don't have one. No extra charge for this.
Most places that sell them can do this for you. It takes a tester designed to put them under load and evaluate their performance.
Most mobile techs should have a load tester for batteries. It can be a bad connection, especially the ground lead and he (or she)can check that for you as well. Or you can clean them up and see if the problem resolves itself. It may be a loose or dirty connection AT the fridge as well. If you have a small digital multimeter we can take a reading AT the leads into the board and see. Do you have a Service Manual or not?