We really like winter camping -- the cold weather holds the crowds way down!
The Achilles heel of most trailers is the outflow tube under the water tank -- it's thin and freezes sooner than the rest of the water system.
If it is in the high 20s, we usually don't worry.
If it is in the low 20s, we use both of the tricks described in these blog posts:
Heat cable blog postHot water recirculator blog postThose tricks work till about 15 degrees. Then they don't. So that is our practical limit.
We just got back from ten days of sub-freezing camping in the Sierras -- no problems at all.