Someone here - my apologies as I can't remember who - used a product called TKO Orange to clean a water tank that had accidentally been partly filled with some diesel
. He reported that it took a fair number of fill/flush cycles but it worked.
You've got a tougher challenge but it might be worth a bottle of the stuff. If it fails, just go back to using it for diesel
After the post, I bought a bottle and now use it for many boat cleaning chores and as a bilge
cleaner. They claims it contains no toxic or hazardous ingredients and includes orange peel. That must be some nasty orange they use as they say it can strip tar or clean oil
Here's the TKO Orange web site: HOME