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We pulled the tank, which is the only way to really do it, so you can invert it to drain it completely.
Clean out what you can (and dump it with the other oil waste) then use detergent and water, hot water if you can. Liquid Tide is very good at dissolving oily waste. LesToil, etc. also work fine. Just wash it out, and repeat. Fill the tank partway and shake it to help get the crud dissolved. Then eyeball it and see if that was enough, it often will be.
We let it air dry overnight, that was enough but if you are in a hurry a hair dryer, etc. would do the job in a shorter time.
If you need to "scrub" where you can't reach, coarse salt (deicing rock salt or koshering salt) in a dry tank acts just like sand, and it can be washed out 100% with water after it has done the job.