Hello. Consider trying Krud Kutter -a spray bottle. Your tub will look new, as does the siding on my home [removed mold/mildew] my s/s kitchen sink, tile floor etc. It is non toxic and has very little odor to it. One of the best cleaners around.
Here's a post that might help - https://www.doityourself.com/stry/how-to-clean-a-plastic-bathtub
A magic eraser and a foaming cleanser like Kaboom or Scrubbing Bubbles are all you need.
Scrubbing Bubbles cleans my bathtub very well. Happy with results. The secret to using it is to spray it on and walk away. Wait at least 15 mins then come back and wipe it all up.
A Mr. Clean Magic Eraser followed by a wipe down of Windex on a soft t-shirt