You have just come under the curse of the fiberglass shower floor. It is possible to escape the curse, but only with a great deal of effort.

First I used everything you did. Nothing worked. Then I took a Mr Clean Magic eraser ( actually about five of them) and scrubbed the living daylights out of it for about 45 minutes with cleaner ..I think Ajax. Then I bought Kaboom and sprayed that on. Only leave it for about four minutes, not any longer because it will make things worse,and put it on very heavily. Then rinse it off. The stains that appear will go away as it dries. Takes about six hours.

Then do the same thing all over again about one or two times a week. You don't have to scrub for quite so long. Once it really stays white for about three days, you can just use the Kaboom for awhile.

Goodluck! I hope you can escape the curse of the fiberglass floor.

Ew. This is fun. When we bought our recent house, I tried everything. Harsh chemicals, etc. Here are my notes:
-Clorox Bathroom Cleaner: Smells aweful, cleans nothing, use gloves!
-SoS Pads: No Smell, Cleans amazing, don't scrub too hard, use gloves.
-Comet: I hate the smell, cleans okay, can't scrub too hard or it will take that finish off that helps keep most of the grime at bay (same with SoS Pads)
-Murphy's Oil Soap diluted in water: Smells great, cleans a good bit but doesn't work on stains, no need for gloves.
-Fantastic Orange: It's Fantastic, smells nice, cleans wonderfully without too much effort, no gloves needed, no harsh chemicals. Use a rough rag or rough side of a sponge with this, rinse, then use a harsh floor brush and clean bottom by hand. The longer you leave it on, the less elbow grease needed =)

Tip: After you're satisfied with your cleaning efforts, purchase car wax. Either spray on or wipe on. Apply thin layer to all walls (even shower door). This helps keep mildew and soap scum from building up as quick as last time. It's shiny!.... no, really, I saw this on hgtv...

Happy Cleaning

Use SoftScrub GEL in green bottle. Wipe on, leave for 5 - 10 mins and then scrub with scotchbrite side of spounge, rinse throughly. Also works on fiberglass tubs. You will be amazed!

I used lots of baking soda, a sponge wrapped in nylon scrubbie material, and plenty of elbow grease. It worked!I sprinkled the baking soda over the floor of the shower, wet the sponge, and started scrubbing. I had to rewet the sponge many times to cover the whole area. When I finished scrubbing the entire area, I placed a plug over the drain, and sprayed vinegar all over. I let it fizz for a while, and then rinsed everything down with water. I had tried a lot of other products beforehand with very little luck. The baking soda did wonders!

I have one of these dreaded shower floors and I have found that Bar Keeper's Friend or Bon Ami work great when combined with Magic Erasers. Soft Scrub with bleach works too, but the smell is overpowering at times. If anyone else has found something that works, please post!