IF you don't want to leave your rig plugged in all winter I suggest
the Battery Tender Jr. I have one on my truck, on my motorcycle and one
on my trailer. They're fully automatic and won't over charge and make
your batteries use any water. (Check water every couple months anyway
DerbyCycles.net have the 6 or 12volt models for $25 with free shipping.
That's the cheapest I've found online or in a store.
I do not work for them! They are in the bluegrass state however
My truck battery is still the original going on 8 years old. It may strand me
tomorrow but today it cranks as good as new!
You really can maximize battery life by keeping them fully charged.