My batteries were install in 2008/2009...
I get the same result leaving my batteries on my OFF-ROAD RV. They are totally disconnected however...
I fired them all up a couple weeks back after sitting since October last year unattended. Good for yet another season... These are the standard issue GP 24 Interstate batteries that usually come with the new trailer.
I started out with four 85AH installed but I lost one right away in 2008 trying to keep the trailer plugged in all the time and it had a single mode converter/charger. Boiled the fluids out of one battery and it shorted internally. After that I installed a 4-stage converter/charger unit which did great from there on. Now here lately I have just been switching my batteries totally out of the circuit and let them set unattended for the winter season. They were charged to a full 100% when I disconnected them in October and last week when I tested them they were at 12.4VDC.
They are back running off my trailer converter/charger now
If I get my planned 120WATT Solar Panel installed for evaluation this season that will become my battery tender over the winter months
Roy Ken