Yes, there are always at least a couple of gallons of water {added after a thorough flushing in both the gray and black tanks} so adding a couple of quarts of antifreeze is about right. The black and gray tanks are only 20 gallons each and the tank heaters are not huge. I just use them to take the edge off of the low temps.
The space heater is plugged into a GFI and has a kill switch if it is bumped much less knocked over. I had an electrician install the dedicated 30 amps with its own box just below the house breaker panel. Normal snowfall up here is about 23" a year spread over half a dozen 2 and 3 day events, no big deal but... last January we had 2.5' of heavy wet snow with temps in the mid to low teens which while unusual snow {like "stuff} happens".
Here are a couple of shots taken last January:


:E