I've been in those temps when we had our travel trailers (Sunline, Winnebago Minnie) and never had a problem. I would open cabinet doors where I knew there were water lines running behind, near or in the cabinets and leave the furnace on overnight. In some cases, if it was getting below freezing for several hours, I would disconnect the fresh water hose.
If you're really nervous about it, just blowing out the water lines is very easy but you have to bypass your water heater unless you want to drain it....which wouldn't be necessary for a few hours of below freezing temps.