Dannye, as it appears you're already aware, its hard on a battery to sit for an extended period in a low charge state. While cold weather isn't such a problem so long as a battery is charged, if they are discharged that's when freezing can become a problem.
As far as Battery Tenders and Maintainers and Trickle Chargers and brands and types and sizes are concerned, there was a long discussion some time back that covered that topic ad nauseum. There are tons of OPINIONS (some professional some lay) about the size and amps and rating and brands and makes n models and how many batteries can be coupled to a particular charger etc etc etc READ AND DO YOUR HOMEWORK AND PAY YOUR MONEY AND MAKE YOUR CHOICE LOL
That being said, I would probably at least remove one battery cable to prevent any discharges,,,,,,,,,make sure they do not sit long periods since they self discharge even if new and perfect and were initially charged just fine,,,,,,,,,if they become discharged DO NOT LET THEM BE EXPOSED TO FREEZING,,,,,,,,study the tenders and maintainers and make an informed decision based on the specs and consider all the opinions offered here, take or leave them at your peril lol I have seen one person swear by and recommend a certain brand, size or type, while another claims they are junk, and I'm sure not going to call either a liar, I enjoy all opinions and stories and preferences, they may be right they may be dead wrong, but all are entitled to their opinions and experiences in my view.
Best wishes
John T