If your rig is plumbed using PEX, the danger is significantly reduced. Although the manufacturer won't quite come out and say that PEX is freeze-proof, the fact is that it is quite robust to freezing. It is really the fixtures, traps, and dump valves that are at most risk. If the fixtures are all in heated areas and you keep antifreeze in the waste system, you have significant head-room. Often, the long pole in the tent is the outdoor shower. I am currently working on making our rig 4-season, and that fixture is one of the biggest issues. I am thinking of adding valves so that it can be separately winterized.