I found this newspaper clipping taped to the original owners manual for my 1978 Trillium
1300. It says,
Tell Margaret Sweet (who requested aid for cleaning her fiberglass bathtubs and showers), that she will be amazed at the results if she'll use any of the liquid fabric softeners on them.
Pour fabric softener full strength on a wet sponge, rubbing it on the fiberglass while it's wet; let stand a few minutes; rinse and shine. The fiberglass will sparkle."
I have no idea who Melba is or was or Margaret Sweet either, but this sounds intriguing enough to me that I bought a bottle of fabric softener to try it out. I'll let you know the results. If anyone else has heard of this, chime in with any info you have.