I use the pea gravel method, with a STRONG mix of water and liquid detergent, like half a bottle of Dawn to 3/4 tank of water. Strap the tank down tight in the back of a pickup, and drive around for about a week. Another good cleaning solution is Trisodium Phosphate, the stuff that's used to clean oil stains off of concrete. About 1 or 2 cups of TSP to half a tank of water, plus the pea gravel inside will give it a really good scrubbing. Turn the tank over occasionally to make sure it gets scrubbed top and bottom.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!