I suspect your 2001 model single mode on-board converter/charger unit has boiled out your battery fluids over time and then the winter froze up the battery.
The older converter/chargers only put out 13.6VDC and this is just right on the edge of boiling out your battery fluids over time. I lost one the same way. I had to start inspecting the battery fluids at least every two weeks to make sure they was not boiling out fluids..
I eventually replaced my older converter/charger with a smart mode version. I went with the PD9200 series units.
Just a couple of my theories...
Roy Ken