From what I have read that 6 volts are a weak source, to begin with, and the long storage exacerbates the situation.
My 6v Optima sealed battery holds a charge over the winter "rest period".
That Optima 6v is over 17 years old (I bought it in 2014).
I have used a 6v trickle charger on that one (when I had not started the engine in a year)- I do not know if that was needed.
One of the trucks I sold/traded has an Optima 6v that still holds a charge - it was bought in 2006.
