My wife's g-pa's RV is parked at our house, and I went out and checked it a few weeks ago and the battery was completely dead. So dead that I had to hardwire a 12V power supply to the terminals for like 20 min just to get the voltage above the threshold where my battery charger would even see the battery and start charging (I think it requires something like at least 3V before it will start charging just to make sure you didn't try to charge after accidentally reversing the polarity). I charged it for over 24 hours at 15 amps. But I checked it again on Monday and the batteries were completely dead again.

These batteries are new- the sticker on them says June of 2019, so less than a year old. I pulled it out of the RV and hooked it up to my charger that has a reconditioning cycle that supposedly can reverse some sulfation. The reconditioning cycle ran for 24 hours, but then it still threw a fault code for an open cell when I tried to charge it after that. The instructions say that it might require multiple 24-hour reconditioning cycles, so I've got it going again.

Are these batteries just toast? They really don't like getting completely drained. I might just tell my wife's g-pa that he should try to take them back since they probably still fall under Costco's 12-month warranty for new batteries.