How do I whiten my faded tub surround?

What is it made of?

Vinegar

There is a paint that is made for tubs that I have used. They do require 24 hour drying before using and some prep time, but will come out looking like new.

If it's faded and you tried to clean it with no change then the finish has worn. You can use Rustoleum or Homax tub and tile paint kits to paint it.

If it's just faded and dull from age I'd try giving it a good scrub with baking soda. Using the shower head, wet the tub & sprinkle on a light layer of baking soda. While it sits, mix up a hot bucket of water with a squirt of Dawn. Now scrub it really well. Rinse well. Once it's completely dry, rub on Gel Coat fiberglass tub & shower polish. Buff it til it sparkles!

You can try an auto whitening polish

Use magic erasers with a paste.

Magic Erasers: https://amzn.to/2WBpyW6

Paste Cleaner: https://amzn.to/2HUg4Sz