Costco 6 volt batteries- both died

next time, install a battery disconnect switch so all you have to do is turn the switch when leaving it.
and as others said, any battery allowed to go completely dead, for a long period of time, will likely be useless.
even a Trojan battery would likely be shot.

Died... but not of natural causes or defect. Costco may give you a pass but consider some solar to keep them charged. At least a battery switch to make the disconnect a bit easier.

Try using an inexpensive dumb charger from HF at 2Amps for a loong time. You may get lucky.

Most often when a deep cycle battery gets discharged below 50% charge (apprx 12.0VDC) and do not get re-charged back up right away they will start self destruct inside the battery...

You need to have a multimeter reading the battery terminals when you are charging as also sometime when a battery is really low the battery charger will kick off and not keep appling DC charging voltages. Sometimes this has happened to me and I had to start out with a lower charging setting (like maybe 2AMPS) and sort of work my way up in small steps to keep the charger from kicking off.

A couple of weeks of trailer lights left turned ON is probably a death warrant for your two 6V batteries. When all re-chargiing fails of course take them back and maybe you can work out a replacement deal with costco. They sell alot of Golf Cart batteries... I don't believe they are at fault however based on your failure description.

Try lower charging setting and use a multimeter across the battery terminals and make sure it stays in charge mode for a couple of hours and hopefully the batteries will start returning to normal... I don't know anything about our brnd of charger but it would probably work better if you could each 6V battery separately. DO NOT however apply 12VDC to the 6VDC battery terminals. I think about 7.5VDC is the norm charging voltage...

Like they always told me at work when I did something like this... "Bend over Braddy" and then each fellow technician gets a free boot shot...

Let us all know how you making out... Those GC2 Golf Cart batteries are built to withstand alot of punishment...

Roy Ken

Lead acid batteries do not appreciate zero voltage. They are most likely permanently damaged even if you were able to exert some charge into them.

I would say that running the batteries completely dead was the fatal mistake. Deep cycle batteries are designed to not be discharged below about 50%.