How to best clean a front load washing machine with mold/mildew. ?

My husband is a heavy equipment mechanic and is not provided uniforms. His work clothes are always covered in grease and diesel fuel. He has enough pairs that we only have to wash his work clothes once a week and we always run the clean tub cycle after washing his clothes. But there is build up in there and I have tried everything to clean it. We always leave the door open to air dry and use the tub cleaners and bleach to try to get it clean. Any suggestions on how to clean it? Also, most of the build up is on the rubber part in the front.