Sic - I am definitely jealous of the amount of space you have there! Is that greenhouse a 16 x 24? Have you looked into doing the double layer inflated sidewalls? From what I understand it can make a huge difference in insulation factor for not much more cost. One trick I use is Reflectix on the north wall of my greenhouse. I think it cost me about $100 but I easily save that much each winter in energy costs. My GH has polycarb sidewalls though, so it is very easy to line them with the Reflectix.
The other trick I have learned over last year is running a big fan. It seems counter-intuitive to me, but my big 30" oscillating fan seems to make more efficient use of the heat being kicked out by my 30,000 BTU heater. So far I haven't had to run it full blast and we were below 20 last night. Temps in the GH are staying around 60 at night.
I tried to overwinter about 100 plants last year, hoping that they would give me a early start to my season, but in practice, they started producing at the same time as my new plants. I decided this year to save myself the trouble and the greenhouse space. You being in Florida, you may see completely different results though, and I hope you do.