We recently spent 6 nights in our Maggiolina. We were also very pleased to find it had stayed completely dry over the winter when we opened it up before heading out. One night was spent at about 5500 feet with temps dropping into the 20s, but we were quite toasty and comfortable with just a double North Face 20 degree poly bag.
However, over the winter we did replace the foam mattress that came witth the tent. I purchased the other half of a high density foam block from someone else's Airland mattress project. We had Friendly Foam (U-District here in Seattle) send the foam out to have a high quality quilted cover put on the foam. This is a good bit thicker (and heavier) than the stock mattress and really helps to warm things up.
You might want to consider putting a thin (3/8") closed cell foam insulating layer under your mattress as well Corey. A lot of heat is lost through the bottom shell of the tent.
We really like the idea of the 12V heater though ;-). We would probably just use to get things warmed up so we don't have to get into a cold bed and then turn it off for the night though. The amperage draw is something I'd be interested in hearing more about...