Try heating a vinegar/water mix in the tank followed by sanitizing (cold) with bleach. During the very first trip we took in our TT, I got a tank full of bad well water. I tried flushing, sanitizing, TastePURE... We could always taste and smell it. A year later, I was about to re-plumb all the tubing and replace the HWT. This was my last shot at redemption....I followed the instructions I found here.
I tripled the time it suggested and it worked. It uses a lot of vinegar, luckily my local Costco carries gallon jugs in a two pack. I can finally use the water to make coffee or pasta and not catch a whiff of it in the final product!