Just today, my furnace began running but it's blowing cool air through the vents. I tried turning the thermostat all the way down, but it still runs. I've been living in my mobile home for 15 years and this has never happened before. Not sure if the following will be helpful, but it's a Miller unit, model # MGHA-056ABFC-05. Thank you.

Most thermostates hand a small slide switch for the fan marked AUTO/ON. I think your switch has been moved to ON which would be 24/7/365 until you move it back to AUTO. However, running the fan 24/7 uses only slightly more electricity than the AUTO setting and it will make your space far more comfortable, so I'd leave it on ON.