The house we rent has a white fiberglass shower pan. It gets dirty. Like, so dirty I am not going to take a photo of it to show you.
It is also difficult to clean, because it has grooves radiating out from the drain like spokes. Gunk gets stuck. Even Cadillac has a hard time cleaning it. Tried bleach, Scrubbing Bubbles, Soft Scrub.
Then I happened to see a post for using oven cleaner to clean your tub. It’s apparently important to use Easy Off Fume Free with the blue lid(say other sources, not the one I linked, which though it’s reliable Reader’s Digest doesn’t have this info).
Like this:
I left the Easy Off on for a couple of hours. It says it’s fume free, but it’s not, though I guess it is less toxic.
When I got back, I scrubbed the shower with a plastic sponge. The dirt lifted right off.
I guess it makes sense. Easy Off is designed to lift off grease and baked on gunk, and the stuff in the shower is probably (ew) a combo of skin and grease and soap scum and hard water.
MARGARET DILLOWAY 
Yay, a new tip. I found another good one. I use dishwasher soap the liquid kind to scrub the toilets with. Great on porcelain.
I’ll have to try to Easy Off. Thanks.