Soap, water, and a little elbow grease

Bed Bath Beyond has cleaners in a spray that is for resin

Vinegar and water. Use a solution of about one part water to 1/3 parts vinegar. For really difficult jobs you can use straight vinegar. Undistilled vinegar has antibacterial properties and also neutralizes bad odors.

I have used engine degreaser, it is a strong detergent. Just spray it on, let set for a few minutes and hose it off.