First, not sure what you mean by the release hot water valve. A picture may help.
Second, did you set the bypass valves on the hot water heater for winter, drain the hot water heater? Do you have a winterizing kit on your water pump? This saves dumping 6 gallons in your fresh water tank. Whenever you connect city water to your trailer, the outside shower is also connected and ready to use. If you use your fresh water tank, the outside shower is also ready to use. Therefore, water is always in the outside shower water lines whether or not you use it.
Here is a link on how to winterize a rv water system. http://blog.goodsamclub.com/2011/10/winterize/
If you google how to winterize rv's you will get numerous videos also.
Good luck, be sure your are winterized properly.