I'm sure you are aware of the different ways to retain heat. They work well, up to a point. But, as soon as the temp starts dropping towards freezing, nothing will keep a hoop house warm. If you run a heater in the cold weather, you might as well heat the entire county! The main thing I would do, is build the north wall out of block filled with dirt. Paint it black. Half of the east and west walls should be insulated also. Double covering with a blower is a must. If you did that, you could probably run a small gas heater that would keep your plants growing slowly. As far as tomatoes, peppers and other heat loving plants, look for them to start slowing down almost immediatly, as it gets coller and cooler. I would concentrate on carrots, onions, spinach, cabbages and such. Cool weather crops. Hoop houses are not really meant for prolonged winter use for heat loving plants.
My little greenhouse keeps me in onions and carrots all winter here in Idaho. You can't dig them out of the frozen ground in the winter, except in the greenhouse. I have cabbages, bok choy and such blooming in February.