In truth, what is left in the tank are oils and decomposition compounds, which can be VERY persistent. I second the dawn dish soap, as it cuts grease, and then a good bleaching. (Rinse until no bleach odor remains).

You will probably be able to still smell it if you stick your nose in there, and there's no real help for that....it will fade over time. It's like skunk spray--you can mellow it out, but only time will get rid of it.