Any time I have a plastic or rubber material that needs to be vigorously cleaned, I go for Westley's Bleche White. I think the primary purpose is for cleaning the white letters on OWL and RWL tires. But, it works on tons of stuff that need aggressive cleaning. I don't bother with OWL and RWL anymore, but still keep a bottle on my shelf.
The fumes from Bleche White will irritate your eyes, but it's the burning sensation that tells you it's working.
Good luck.