IF, by some strange happening you did get a load of diesel in your fresh water tank, it's gonna take some real work to get rid of the problem. If the diesel has been pumped throughout the water system, I don't think you will ever get rid of the odor. Hopefully, someone has had better luck than I have had removing the smell of diesel fuel. I do know that clothes soaked in diesel are history. The smell is just impossible to wash out, and no matter how many times you wash them the diesel smell remains.
I just read your last post. As much as I hate to say it, your going to end up in court if the selling dealer is not willing to do the right thing. The right thing in this case would be to completely replumb the coach. New hot and cold lines, new hot water heater, new ice maker, ect. We would sure like to know where you bought your new bus.