We travel in the Winter months out West and it is not unusual to be in temps well below freezing and even below zero.
I keep our fresh water tank close to full and only hook up my sewer hose to drain my black/grey tanks.
I use a temperature controlled outlet, like this(
Thermo plug).
To this I hook up one of those shielded garage trouble lights with a 150 watt incandescent light bulb. With this installed in my wet compartment we have never had any pipes freeze or tanks freeze.
A lot of coaches have ducts where some of the furnace heat is diverted into the wet compartments but we very seldom use our gas furnace. We prefer the electric heat to keep our coach comfortable so the 150 watt light bulb is our solution.