Re: Fresh water tank.
There is bound to be at least a tank drain, usually located near the fresh water tank. There is almost always a low point drain for each the cold and the hot water and there is a drain for the water heater. You can find a good set of instructions on how to sanitize your fresh water system at the link below. http://adventure.1tree.net/index.php/rv-living/maintenance/water-system-sanitizing
We have lived full-time in our motorhome now for ten years and we have been drinking the water from our tank the entire time. We use a good quality carbon block water filter for all water and we use good judgment on where to get our water, but we don't waste our money in buying bottled water, most of which is nothing more than filtered tap water.