I think snowbirding can be pretty darn energy efficient. Depends on how you do it.
For the last four years, I have had neither heat nor AC in my van. Well, I did have a 300 watt heater which I think I used for a total of 30 minutes over the four years :B
My van is powered by an extension cord and a single power strip with a 10 amp breaker. I think I have used two of those little camping canisters of propane over the four years.
I don't drive much at all. 1000 miles, give or take, for the north-south trip twice a year. Maybe another 500-1000 miles of side camping trips. When I am in one place, I get around by bike.
When I'm awake, I'm usually outside the van. In the cool evenings I wear a lot of layers.
I've had two or three days when I have crawled into bed at 4:30 in the afternoon because it was dark out and cold, though! Good place to read and surf the net.
All of this is a lot less energy than I use in my small, energy efficient stick house up north, where I run heaters in the winter and sometimes AC in the summer.