A 3 stage charger is good. If you do decide to leave your trailer plugged in, just check the battery level every now and then to be sure. I leave mine plugged in most of the time. I unplug every now and then to let the batteries run down some before plugging it back in to charge them back.....don't know if that helps the batteries or not.
A charged battery should not freeze. It got down to below zero several times last year with no battery problems. An uncharged battery will freeze, so just make sure those puppies keep a good charge.
Chap , DW Joy, and Fur Baby Sango
2006 Ford F150 Super Cab 4x4
2008 Surveyor 263