You should be draining your tank using the drain on the underside of the tank. To flush, drain, fill, drain. You would only need to use your water pump to flush the water lines.
You should be draining your fresh water after each use, if it will be more than a few days till you use it. If you do that, generally you only need to sanitize once a year at un-winterizing (or less).
Doug and Patience and Shelby the mutt
2015 Crossroads Sunset Trail Reserve ST26RB
2013 Ford F-150 3.5 EcoBoost XLT Supercab 4x4