The furnace could have a cracked heat exchanger and needs to be checked out. Not a quick test either, you want it tested when it's running hot too.
Check out the detectors and replace if needed
The one time we had electric we used a small bathroom heater set to low on the stove so it wouldn't be tipped over. Worked great but I'd use a oil filled if I had the room.
The one time I used a portable propane heater I'm really glad it was during the day. They don't play well at high altitude where the O2 is thin. I became ill even with several windows and a vent cracked.
Our only choice was to set up the TT so that we could run the furnace as needed with solar and extra battery.
