I keep reading "house built in the 40's
Has insulation been upgraded? how much of what type insulation is in th attic?
Windows as you described can be completely sealed, but a thin piece of glass let's a lot of heat out. Does it have storm windows on the outside? if not, a piece of plastic over the outside would create a insulation barrier that would help keep heat in. Walls is another concern. My Mom and Dad's house was also built in the 40's. When they bought it, after renting for years, they started upgrading. it had wood lap siding. The insulation people pulled boards off, and vacuumed out the old insulation. It was,.....saw dust! They then pumped in insulation. Dad had siding installed which had an insulation wrap included under the siding. He had the attic pumped with insulation as well. The switched to gas heat, gas water heater, and gas stove. his power bill was cut in half, his heat, gas vs oil, was cut by 2/3's. They had a couple of ceiling fans as well. this was on 9' ceilings.
I live in a 14X70 mobile home. it had a electric furnace. It would suck some bucks out of my pocket. I removed it, and made that area a cupboard. I now heat with a kerosene monitor heater, I use 5-6 gallons a week, I also use an oil filled heater in the bedroom. I am comfortable and it doesn't break the bank.