I have not sanitized my water system since we bought it 9 years ago.
I fill it with city water every trip (about once a month), and drain the left-overs after every trip. We don't use the on-board water for drinking (other than teeth brushing). We use bottled water for drinking and cooking.
Only issue I have have ever had is the small clear rubber tube that runs from the tank to the vent on the fill plate sometimes gets gunk growing in it. I remove it and run a bottle brush through it once a year or so. The tank itself looks clean as a whistle - I can see it from my rear cargo hatch or lift up the bed shelf as it is under my bed.
Same thing with our black water tank. I never use one of those wand things to clean it. I just put an enzyme packet in before every trip and fill it full before doing the final dump after every trip.
Steve