You have a 50 amp unit. At least I think you do; if not then all of this that follows doesn't apply. So half of your 120v electric appliances are on one side of that 50 amps and the other half on the other side of the 50 amp circuit.
You need to find a way to split you electric usage between the to side so you share all your loads equally. Like - coffee put on one circuit, heater on another.
It is likely that ALL 120 outlets are on one leg of the 50 amps, but I'll guess that isn't the case. You need to figure out what circuit is on which leg.
That floor plan also offers washer/dryer prep. So each of the outlets for those appliances are on separate circuits. Or are in my case. So you can plug into one of those outlets so your heater isn't on the same circuit as your coffee pot, electric skillet, etc. Your furnaces is 12v so that doesn't enter into the equation here at all.
Again, figure out what each circuit (circuit breaker) powers and share the load.
As for a smaller powered heater. I'm not sure. Most of them seem to be 1400 watts (a full single circuit worth of electricity). It's how they can keep up with your heating load. If you get a small heater you will probably only wish you had 2.