If it's down near zero, all it takes is an hour for the valve to freeze up. Then we need to heat it with a torch just to open it. Even the outlet pipe cap will be frozen on. At 20 degrees or less, we need to either hit it with a mallet or dead blow hammer, or hit it with the torch as well. If the vent freezes up, and we don't know it, bye bye trailer as it will implode when unloading.
Plain and simple, in the summer, water hauling is a great gig. In the winter it kind of sucks...