Need some technical advice

2007 f250 6.0 lariat. As of a few days ago truck was running/ driving down the hwy and my tuner showed my battery voltage dropping drastically at around 6volts. The truck died towed it home charged the batteries and replaced the alternator drove fine for 3 days and died on the way home again same way. Nothing added to the truck no lights were left on just all of the sudden voltage tanked. Took batteries to oriley and came back bad went to shop where I purchased them back in February for a warranty swap and they said they tested good and charged to 12.99 volts. Got them home and upon starting truck batteries are sitting at 11.5 volts. Do I have bad batteries or is it something else?