I have a nortron electric furnace in my mobile home. It has always worked great, but lately when the temperature goes below -18C it won't heat up to room temperature and runs constantly.

That is understandable. Your outflow BTU is greater than your heater BTU can produce. Unless you have an exceptionally well insulated roof, wall and floor you will not keep the house warm. Mobile homes do not typically have the above noted insulation. Are all walls skirted and well sealed? -18? Hate to see your electric bill. What is the KW rating for the furnace?