I got this tip many years ago from a Trailer Life RV Tech Newsletter and thought I would share with everyone as I just did this today to my trailer.
To de-winterize and sanitize your fresh water system I do the following:
I turn on the taps and run the antifreeze out of the system. After doing this I reroute the water so that it may fill my hot water tank in the following procedure. Than I sanitize my fresh water tank and lines to kill any bacterial buildup by doing the following:
Drain the water tank completely, then refill halfway with clean, fresh water.
Mix 1/4 cup of household bleach for every 15 gallons of tank capacity in a container with a gallon or two of clean water.
Pour this mixture into the water tank.
Top off the water tank with fresh water. Drive the rig around the block to mix the solution.
Pump water through each faucet so that all the lines are filled with the water/bleach.
The hot-water tank holds at least 6 gallons of water. Run the hot-water faucets until this much solution has passed to insure that the old water has been purged from the hot-water tank, and it is now filled with the water/bleach solution from the water tank.
Let the water stand for several hours.
Drain the entire water system, hot-water tank included.
To remove the bleach odor, mix 1/2 cup of baking soda with a gallon of water and pour into the freshwater tank.
Fill the tank completely and pump this solution through the water heater and the rest of the water lines. This solution can sit in the system for a few days. Driving the rig around the block will slosh water around and thoroughly clean the tank.
Drain the entire system and refill with fresh, clean water.