We have had good success with a good soak 2 or three days with a good caustic de-greaser,,wild dog, purple power or what is available in your area, Use it full strength, seal all openings and rotate it occasionally. Then wash it well inside with a hot water pressure washer. You probably won't be able to get to it all due to the baffles, thats OK, rinse well, very well with clear water and dry for several days. We use a heat gun to make sure its dry inside. Use one of the additives to keep slime, barnacles, and other critters away.
Good Luck, Mac