I noticed that they didnt say anything about removing the batteries and keeping them in a warm place over the winter. I have always done this. Is it necessary??

FYI,

2 years ago I had a year with only one time out on my boat. A 2001, I did not charge the deep cycle battery either so I wound up losing a cell in it over winter. Not sure why I lost it but i suspect either deep cycle sensitivity or lack of charge. So now I take that one in but have been ok so far with the cranking battery since 2001.

the cells could freeze and if you put them inside keep them off the floor . it could pull the charge from the battery.