Years ago, I tried using the TT in winter as an "addition" on the house. I covered the area under the TT with bubble-wrap concrete blanket (used by builders to keep concrete warm during winter curing)skirting. Then I built a short "air-lock" to attach the entrance to our enclosed deck. Then I ran an electric heater inside. As soon as the temperature dropped below about 45 degrees the electric heater ran continually and when it got colder outside it required the propane furnace to come on every so often. This was just an attempt to keep the inside of the TT at 60 degrees. I gave up after one winter because it was too expensive.
An electric heater (1500 watt) running for 24 hours (at an electricity cost of 15 cents per killowat-hour) will cost $5.40 per day to run. This is a lot of money.
Steve