I have to repeat what Hunter11 said. I always fill my water tank at home with well water that goes thru the water softner. Can't have bad hair days while camping ya know!!!
Anyway we have periods of a week or two that we do not use are camper so we always have to drain the water tank and the water heater. I learned the hard way that if untreated water sets for more than a few days it will smell like sulphur everytime. I also put a small amount of bleach in the water tank before I fill it just to get any critters out that might try to grow.