The furnace shoüld keep up, so there is issues. As stated drop a partial side of the colorplast and take pics or video down vents. You'll find it. You've said bad air flow at a particular vent, theres your starting point.
I also use the electric heater trick, but I find it better for the furnace to get it to temp then the electric heater keeps it the rest of the night. Ive froze all night for the electric to get it to temp
If at a campgrounf with electric, the eletric heat is free, while propane costs you.