Mostly travel (Denver, Seattle) and camp in winter. We do not add anything, I keep the fresh water as full as possible and don't drain the grey/black until needed. We use the fridge. We are in below freezing commonly, we do not have heat pads. Never had a problem.
One thing we do different is we ONLY use the furnace (70-74, cycles about 20 min on 20 min off at zeroish, we also updated to the digital thermostat, it cycles better). If you use supplemental heat it will not run the heater enough and heat does not get to the tanks.
BTW we use bottled water for drinking.
Enjoy camping in snow is awesome! Make sure you have chains for TV.