Have never seen one without a drain on the fresh water tank somewhere. There are recognized ways to sanitize such a tank - search on the Forums for "sanitize" and you will find them. They involve adding ordinary household bleach to the partially-filled tank, then filling it all the way with fresh water, running this solution out of all taps, shower head, etc. for a bit using the coach's pump, then letting it sit for a pre-determined amount of time, then draining (you can always use the water pump to pump out the tank) and refilling. Most do this every year and sometimes more often.
Good luck - and welcome to the Forums.