Several schools of thought on the subject here.
Remove them to the cellar or garage, check the fluid, charge periodically.
Leave in the trailer, check fluid, fully charge, turn off the 12v switch or disconnect the leads. Periodically charge.
Batteries are fine outside for the winter as long as they are fully charged and the fluid stays where it should be.
I used to pull them till I read they were fine to about -60 on the Interstate website.