I have 4 space heaters, I added a pair of special HEAVY DUTY outlets (15/20 amp) to handle two of them, one plugs into the washer/dryer outlet and one to a special independent power line.
In the wet bays I layed a string of C-9 Christmas tree lights (NOT LED LAMPS) under the fresh tank, this string is aroud 250 watts, and a 100 watt lamp in the waste end of the bay.
I had one line (A cold line) free when it hit 11 degress last winter.. I plan on re-locating some of the C-9's to prevent that.
Oh, I added a 150 watt space heater to the waste end that night too.
All the other plumbing is inside the RV, works well. for me.