The WFCO would have stayed at 13.6 or else dropped to 13.2 like it is supposed to after a couple of idle days. 13.2 is correct for 80F while 13.6 is right for around 48F.
If the rig sat for three months at 13.6 at anywhere near 50F most of the time, there would be no water loss. You should have been ok leaving it plugged in unless it was really hot all that time.
Water loss can be from battery age, or from being worked hard doing a bunch of 50-80s, but this was not the case. Got to wonder how much water they had going into the storage period and why was that?
I suspect we don't have the full picture here?