I edited my post - batteries were brand new 2 years ago.
They actually read 6+ volts. My volt meter doesn't allow me to read as exact as 6.4 volts, but that looks about right. Maybe I need a new volt meter.
When I removed the batteries I put them on the floor of my garage. I didn't wire them back together.
I put one of them on a 6V 6A charger but accidentally put the other on 12V, 2A for about 10 hours. Then it put it on 6V 6A. I swapped the batteries off the 6A charger for the rest of the winter, never more than a few hours each.
It never occurred to me to check hydrometer. All my batteries (car and mc) have been sealed for years. But I can look at these to see if they are different. Did not check water level or voltage.
I'm recharging them now via house power (they are in the RV) and then I will disconnect them and check them individually.
I'm an idiot about electricity.
Thank you all.