John,
I know the area well as I spent part of my childhood near Taylorville and still own some farmland inherited from my Granddad near Blue Mound.
Six inches of insulation under the floor, combined with a "skirt" will help.
Blocking the cracks in the slide openings with blankets or towels will help a little more.
Get some 1/4 inch bubble wrap and cover the inside of the windows with two layers (bubbles against bubbles) - it lets the light in and keeps the cold out.
Each one of the above will make a small difference but all of the above will make a big difference.
e.i. "The whole is greater than the sum of its parts".
BTW - My heat sources:
Primary - 1500 Watt oil filled radiator, controlled by a Lux WIN100 thermostat.
Secondary - 1250 Watt Utility Heater, controlled by another Lux thermostat.
Backup - the propane furnace.
Primary thermostat is set for 75°F
Secondary thermostat is set for 72°F
Furnace is set for 68°F
Last winter, the furnace ran for about 15 minutes total time.