We washed our covers with cold water in the "hand wash" cycle of the machine. Used Tide and Oxyclean, usually wash with homemade laundry soap. Covers came out clean and nearly dry. Hung overnight, then stuffed the next day. The dinette back cushions started to come apart at the corner, where they had been next to the window in the sun. Other than that, they came out perfectly clean and smelling nice. A couple of the metal zippers were corroded, little lube and gentle work with pliers and they're fine now.
Next time I will probably try the kiddie pool. When these fall
apart I'll make my own covers. My foam was fine, no stains or mold spots. Left the foam in the sun for 3 days, turning every few hours when I had to move them to keep them in the sun.
We used to clean car interiors with a cleaner called Maalco or something. Very harsh, but would take just about any stain out, worked great to remove smoke residue from headliners and plastic too. Applied as lightly as possible, then scrubbed, then vacuumed with a big shop vac. Either parked it in the sun with the windows
cracked, or left in a heated bay for a day. I did something similar to the carpet and rat fur in my Scamp