I sanitize my tanks and lines a minimum of once a year, usually when I de-winterize. If I fill with questionable water during the year, or the trailer sits for any length of time I will flush again.
My procedure is as follows; With the Hot water and fresh tanks empty I add a cup or two of bleach to the fresh tank then fill the tank. I then fill the HW tank with the bleach water solution and run all taps, showers and low point drains until I smell the bleach. I then leave it sit for 24hrs and drain the tanks then refill and flush the system twice.
We always drink from our tanks and have never had any issues.