When camping and winterized (and our Class C doesn't have a lick of protection: the black and gray tanks are exposed, along with all the plumbing for those) - we use the RV toilet when truly necessary, but flush with about a quart of antifreeze. Have camped when the temps were down in the mid-20's without any problems. Here's a pic of our 5th wheel, which I am currently getting ready for cold-weather trips with Reflectix over the windows:

The Reflectix, with the shades pulled down over it, helps keep the RV nice and warm since windows have such lousy R-values. I also have Camco vent pillows stuffed in the vents. With this same setup in our Class C (plus a quilt separating the driver's area from the rest of the "house"), we were able to stay adequately warm just using an electric heater (the furnace is so loud it keeps me awake)