We have been fulltiming for 3 years and have not sanitized the fresh water tank since before that time. We use it regularly to ensure the water doesn't go stale, and we use only a sediment filter at the water inlet as we want chlorine in the fresh water tank. (we use a drinking water filter under the galley sink for drinking and ice maker).
When we were using our coach only part-time, I would sanitize the fresh water tank once a year, normally in the Spring when de-winterizing the rig. I have my sanitizing checklist on my website at http://www.pjrider.com/RVChecklists.htm
The checklist provides info for using either chlorine bleach or sodium dichlor, but the only thing I have ever used is sodium dichlor (the same stuff I used for chlorinating the water in our hot tub when we had a stick house).