RV Water Tank?

My husband and I have a 34' travel trailer that stays permanently on a campground. We have a 257 gallon tank that we use for water storage, that sits outside, which we poured bleach into so the water does not get stagnant, however we filled the water tank in the trailer before adding the bleach. Returning two weeks later (after a very hot spell) the water in the tank went bad and produced a very bad odor. We flushed the tank out and added the bleach water to the tank, but the odor is still in the lines. Any suggestions as to what we can do to get rid of the stagnant water odor, and the exact amounts of bleach, chlorine, etc we should use to avoid this happening again? Any suggestions would be great.