As long as we keep the fresh water tank filled to from 3/4 to a full tank, we have had no problem with freezing down to -5. We keep a 100 watt bulb burning in the wet compartment on a thermostat controlled outlet, which keeps the water lines and pump from freezing.
As far as the waste tanks go, we have never added anything to them other than the waste. Never had on freeze up since we are constantly adding fresh "waste" to each tank. When they get full, we hook up and drain the tanks rather than to keep our sewer hose connected at all times.
We very seldom run our furnace since we have other heaters to warm the coach when hooked up to shore power.