[f6bits]

Something could be left on and draining the battery. It could even be something you never see, like a storage compartment light. I believe that the fuses are for the 12V devices, and removing them would eliminate any drain on the batteries. But that’s just a hunch.

What trailer do you have? Mine has a new charging system which works pretty well. Some trailer converters are pretty primitive for charging batteries and do a poor job of topping off the batteries.

The other possibility is that one or both of your batteries are faulty. When you do your voltage test, completely disconnect the batteries from the trailer and test them one at a time, then measure total voltage.

EDIT: Oh, I just read you’re using an external charger. You’re charging both batteries in series at 12V? Have you tried charging by plugging into shore power and letting the trailer charge the batteries?