In Canada, we have a policy that chemicals have to have an MSDS (Material Safety Data Sheet). That would give you a much better idea of how toxic the contents were, and how many "forever" chemicals might have been in there.
That said, you also need to know if the company that used the chemical has a policy of "cleaning" the tanks out, in which case, the tank may have a mixture of leftover product *plus* leftover cleaner.
One option if you can get a bunch of sawdust or wood chips is to take a page out of
Paul Stamets' work and grow oyster mushrooms in the tank. They seem to be really good at breaking down nasty stuff into its component parts. For example, diesel oil spilled on your dirt is "bad", but it's 75% C10H20–C15H28 - mushrooms can break it down into those H's and C's. However it has other stuff as well - like Benzene (C6H6 - more C's and H's) and Styrene (C8H8 - more again). They're nasty the way they are, but if mushrooms can break them down, they become harmless. However, other chemicals may have things like lead in them. Those I'd want to grow sunflowers in and then send the sunflowers to the dump.