From the Trojan web site.
'Can a battery freeze?
The only way that a battery can freeze is if it is left in a state of partial or complete discharged. As the state of charge in a battery decreases, the electrolyte becomes more like water and the freezing temperature increases. The freezing temperature of the electrolyte in a fully charged battery is -92.0oF. At a 40% state of charge, electrolyte will freeze if the temperature reaches approximately 16.0oF."
I leave them in the trailer fully charged and disconnected from any load.