It's not hopeless.
Clean, clean, and clean again.
I hope you have an access panel into the tank. Open it and wash with hot water and Dawn dish soap. Rinse well and wash again. If you can reach a
power washer in there, even better.
When you have the tank as clean as you can,
pump more soap and water through the system hoses. Then rinse, again.
Finally, install a carbon filter on any
drinking water outlets to get the last traces out.
You'll live. You're not the first. More people than you might imagine have pumped
diesel fuel into the water tank.