I've got a standard IBC(tote tank) that used to hold diesel; the inside is pretty nasty, not just stained from diesel but near the bottom appears coated with contaminants. I can't tell you a way that works well, but I can tell you that trying to clean it off with an air compressor is not very effective, and scraping/scrubbing through the small top opening really sucks... at least the air compressor was good for blowing the loose bits out the drain.
So far I've got the easier half of the gunk off. Maybe following Wayne's suggestion would work if I took it offroading while partially full of sandy water; can't really see any practical way to shake a tank that size without mechanical assistance.
'Theoretically this level of creeping Orwellian dynamics should ramp up our awareness, but what happens instead is that each alert becomes less and less effective because we're incredibly stupid.' - Jerry Holkins