This has probably been said in one form or another, but just in case:

Lead acid batteries will not freeze if they are fully charged. In fact,
the extreme cold will reduce capacity (while cold) but actually extend
the battery's life!

The challenge, obviously, is to maintain a fully charged condition over
the winter.

At the very least, the battery should be disconnected from any possible
load, no matter how small.

A small solar charger would probably keep up with a battery's rate of
self-discharge.

An interesting question is whether batteries should be left connected in
parallel or isolated from one another. If left connected, a bad cell in
one will quickly discharge its partner. However, under some
circumstances a battery with a relatively high rate of self-discharge
could be "kept alive" to some extent by a parallel-connected partner.

A discharged lead acid battery is likely to freeze with unfortunate
consequences.

Chuck