Electric water heater also a draw
When we have 15 amp power, we usually use gas for the fridge and the water heater as well to keep from popping breakers; that lets the furnace fan get all it wants, and we turn off the furnace when running toaster, microwave, coffee grinder or coffee maker - and we run only one of those at a time. If the furnace isn't on, we'll run the water heater on electric.
You want a 10-guage or larger (smaller number) extension cord if you're running it 100 feet - that will run you about $100. We use a 50-foot 12-guage extension cord when sharing 15-amp power with others at rallies and caravans.