I usually wash with soap and water, then fill with bleach and water to sit for a day or two, wash one more time. This even worked when I forgot to unload a cooler from the beach with beer, bread, and melted ice. It had been so long even the maggots had died and hardened.
Used simple green once and the cooler never got rid of the green smell. Gave me a gag reflex every time I'd used it.