I am surprised your refrigerator is not wired to a dedicated breaker. Ours is. I believe all appliances should be. I would check to see if you have any duplex outlets that are wired to an independent breaker and us one of those, if possible.
We use this heater - - left over from our boating days:
WEST MARINE Portable Cabin Heater | West Marine
Great heater that has kept us warm on some very cold, blustery days - - both on the water and in the tt. Adjustable fan, adjustable heat settings within 3 separate power setting - - 1500W, 900W, and 600W. Plus it has an "antifreeze" setting that turns it on if the temps drop to 38 degrees F. Oh, it has a thermostat, too, so the fan turns on and off automatically to keep heat you set constant.
A bit pricey at West Marine (if they claim it's for a boat, the price doubles
) Cheaper prices available with some googling.