When I lived in Michigan and Pennsylvania, during the winter months when it was 30° outside, my furnace ran all the time to try to keep it 70° inside. In a manufactured home, I think simple air leakage and the simple fact that it is a manufactured home are probably the bigger problem. Manufactured homes don’t have a lot of insulation in the attic (what attic?) or in the walls, and as those things flex when going down the highway, they develop little cracks here and there. I’ve never seen anyone go through the home once it is sited to plug all those cracks.
I am guessing that this is a mid-efficiency gas furnace because of the fan on the burner. The fan looks to be connected to a flexible duct that runs up into a 3" duct. Does that go outside with the flex inside? What am I seeing there?