I am always dry camping and when doing so in the winter it is similar to how the OP does it, the only exception being that I can use the cassette toilet- just use RVAF or WW fluid in lieu of water.
I will often use the domestic water in winter.  I have added valves to isolate the city water inlet and outdoor shower valves so that no fresh water gets to those components on the exterior wall.  I am able to pull the outdoor shower hose to the inside keeping it in the warm.  Depending on the temp, I will just leave the heater on the lowest setting and it is able to keep things from freezing allowing me to use the shower in winter camping and hunting conditions.
on edit:  speeling corekshun.