Last winter I attempted to insulate my batteries, but didn't finish because I wasn't seeing a noticeable difference when driving less than 10 miles doing my around town driving.
Now I am starting to see the difference between 58F, 63F, 74F, 81F, etc in my morning drives. The past few days it has been about 48-60F outside and my batteries has been about 58-63 depending on the day. So last night my dad an I quickly whipped up a temporary insulated box for the batteries. It is made from 1" foam board that has been cut out and duck taped together, just to see if it works before really trying to make it nice.
Well it WORKED. Last night it was 78F on both temp sensors when we installed it, then I plugged the car into the charger and after 30 minutes the trunk was down to 72F but the batteries were at 92F! This morning, after the batteries had been charged for a few hours and just floating, the box was 81F while the trunk was 63F. The drive to school was much improved from the previous drives and I even had power left when I parked vs yesterday's limping onto campus into my spot.
This website has good basic info on deep cycle batteries, including a section on Temperature Effect: http://www.windsun.com/Batteries/Battery_FAQ.htm