I just noticed my last post got cut off at the bottom.
I wanted to say I cut off all
nutrients on those last forming tomato plants early September and still got a good amount of good tasting tomatoes in late October. So when your plants are forming the last tomatoes for those plants you can also cut all nutrients off.
As for heating I personally worry about propane. A very small wood burning stove with Smoke exhaust going directly outside the greenhouse might be a very good option. They hold heat for a long time and are very cost efficient. Electric heaters do a great job but isn’t cost efficient if the cold lasts a long time your electric bill would shoot up. Either of the 2 you would have to weigh the options but stay away from the propane.