keeping the fresh water tank sanitary over the winter

I know the need for sanitizing a few times during the camping season, my problem is this:

The physical location of the ports on my tank are on the sides, I cannot get all the water out. I know that bad stuff likes to grow in damp dark places and I want to keep ahead of the sanitization curve.

so the question is: Is there a chemical/additive that can be put in the fresh water tank to keep it clean over the winter?