Times have changed, you can get a full fledged heat pump now. Mine will extract heat from the air even when the temperature is in the low 40's. Works much better than heat strips - with the right thermostat, it can even turn on the furnace when it's too cold for the heat pump to extract any more heat.
Mine is a Dometic 13,500 BTU unit.
Today is a gift, that's why they call it the present.