Hi Marta. Even with your power switch off, parasitic loads will drain some current from the battery if left connected in the trailer for lengthy inactive periods. A removed battery without connections will show some discharge in 3-4 weeks. Secondly, it will affect your battery life every time they're discharged >50% (watch out when boondocking). A discharged battery can freeze.
I'll generally agree with yukionna's suggestions. Take out the batteries. Bring them into your basement (I use a firewood carrier). It's okay storing them on concrete -- see
this thread. And definitely put a charger on them about every three weeks.
The coming season will be the 4th year on our original lead-acid batteries. One of my two seems a bit weaker. I'll be very happy if I make it all this season before replacing.