Apparently digital multimeters can go bad.  The old multimeter was purchased 6 years ago at Harbor Freight for $4 and has worked well, it has 7 functions which is more than needed for monitoring battery usage. 

Last fall, after giving each battery a 2 amp trickle charge for 24 hours at the end of October, the batteries were tested one month later, to my surprise the one battery was at 12.2 amps (purchased 4/2011) and the other (a Trojan T-1275 purchased 8/2021) was at 12.29 volts!  Another trickle charge for each battery was given on November 20 and 21.  Two days later the batteries were checked on the multimeter with the readings at 12.41 and 12.55 volts respectively.  The 12v battery multimeter battery was checked (it had been replaced 2 years ago) and it was fine.  So, I figured it is time to go and spend the 'big bucks' and make another purchase on a brand spanking new multimeter, this time the price was up to $ 5.99.  Brought it home and the batteries read 12.72 and 12.81 volts, quite a difference from the old one.  So the old one got tossed.

Update: it has now been almost 3 months since last being charged.  Batteries remain in an unheated garage sitting directly on the concrete floor.  Ambient temperature as of testing this morning is 24 F.  Readings are 12.65v (this battery will be 12 years old in less than 2 months) and 12.72v on the Trojan.  So the discharge over a nearly 3 month span is 0.07 volts on the old battery and 0.09v on the new one.  Good to go, almost in the 'spring mode' to go camping!

Originally posted by gpokluda That 12 year old battery needs to go to a battery museum. That is remarkable.

That is one of the great things of this forum, learning from the expertise of others.  I learned on this forum not to draw the voltage below 12.2 for maximum longevity.  Also by keeping the outside clean of acid residue, posts clean from corrosion as well as keeping electrolyte levels up using only distilled water the battery should last a long time, and it has.  It is a basic Interstate, group size 24 deep cycle.  Am curious to see how long it will last, as it does get used a lot during the summer as we usually boondock.