Adding impurities to a liquid raises its boiling point and lowers its melting point (freezing point). That's why you throw rock salt on your sidewalk to stop it from icing over: the rock salt dissolves into the snow and ice and changes the water's freezing point so that it melts and stays liquid even at "freezing" temperatures. What does that have to do with your battery? Your battery's electrolyte is basically water mixed with sulfuric acid, and at full charge the electrolyte is roughly 2/3 water and 1/3 sulfuric acid which gives it a freezing point somewhere around -25F (-10C). The point being that, unless you live in the far north where it really does get that cold, your battery won't freeze as long as it's fully charged.
So far, so good, now the three sentences of more hard-core science (chemistry) content. Just three sentences. And you can ignore the really geeky stuff in parenthesis.
Lead acid batteries store electricity using a reversible chemical reaction. To make electricity the sulfuric acid (H2S04) in the electrolyte reacts with the lead plates (PbO2) in the battery, which makes one lead sulfate molecule (PbSO4), five water molecules (H2O), and electricity (2 electrons). When you pump electricity back into the battery the process goes backwards, turning water and lead sulfate back into sulfuric acid.
That wasn't so bad, now was it? But what does all this have to do with when your battery freezes?
Remember I said a battery's electrolyte freezes at -25F when the battery is fully charged
? And as the battery discharges it makes five times as much water as it does lead sulfate? That means the battery electrolyte becomes more water-like and approaches standard freezing temperatures very quickly as its level of charge drops.
Which means it's really important to maintain your battery's level of charge through the winter. As long as you don't have any energy-using devices inside your trailer keeping the battery topped off doesn't take a lot of energy. A simple battery maintainer, like the one Kevin K suggested, will do the trick. So will a small solar panel
pointed toward the mid-day sun. Or hooking your battery up to a charger once a month. Any of these will keep your battery safe from freezing and
from developing another battery-killing condition called sulfation