Yep, Easy Off oven cleaner!
Eons ago I managed a high end apartment community and our housekeeper shared this tidbit with me. I have been hooked every sense! Step one, clean with your normal shower cleaner. Step two, spray the oven cleaner nice and thick over your shower. I do this with my marble, tile and fiberglass showers. I let it set about 10 minutes then scrub and rinse. It will cut through any soap scum you have. Note: you can use any brand of oven cleaner. Just make sure to turn your overhead vents on before spraying. I like to think of it as my deep clean conditioner and use it about once a month. Happy cleaning!