+1 on a WaterSaver. I open it at winterizing time, vacuum out the residual, and wipe out (as best I can) with a paper towel. I leave the seal open for the winter to let the residual water evaporate so there's no moisture for growth. As was pointed out, the issue is not freezing.
In the spring, I drop a scoop of pool chlorination powder in, seal the top, and then fill using the outside port, so as to sterilize the system. Nice write-up here:
http://rvbasics.com/techtips/sanitiz...er-system.html
Getting the powder through the port is a lot easier than any other way into the tank. Once I've resealed the port in the spring, I don't open it again until winterizing.
If you don't want to install or use a port, you could always re-sterilize the tank when you winterize, then just drain it. The little residual shouldn't break anything, and the sterilization should remove the issue of growth over the winter.