You can search for water circulation at the Forum, should be more than a few posts about it.
The big thing is that no one measure is going to be enough to keep standard trailer plumbing working in very cold weather. You would either have to go to containers of water and drain the water system or adapt a multi-pronged approach to keep your plumbing active. If you assume wrong and a supply pipe breaks from freezing in the Winter, you will have a horrible mess. Skirting the trailer, supplying supplemental heat underneath (like light bulbs, etc), using a heated hose, using heat stirips designed for plumbing, insulating pipes/hoses, and using tank warmers are all things you can try. I believe a lot of folks abandon heroic efforts in extreme temps and rely on containerized water.