The two batteries I own were last charged with a 2 amp trickle charger October 30-November 1.  So here we are 5 months and a week later and the oldest one (12v Interstate, group size 24, deep cycle, purchased 4/2011) reads 12.32v.  The new one (Trojan T-1275 purchased 8/2021) reads 12.41v.  They have remained in the same location all winter sitting directly on the concrete in an unheated garage in northern Michigan.  Today's temp is 33 with light snow.  Guess batteries are good for another season of boondocking without solar or generator assist.