I need to install a grab bar in a fiberglass shower. The shower/tub was installed before the drywall, therefore there is no drywall directly behind the fiberglass. There are no studs available anywhere near where bar in needed. How do I attach the grab bar to the fiberglass shower only? One hardware store recommended the toggle bolt was good enough. Any suggestion? Thank you.

I disagree. I disagree strongly.

Unless you can securely mount the grab bar to some structural member it is better to not have it.
If it is there, I use it expecting it to support me. If it gives way, I loose my balance and fall. If it's not there I won't be depending on it.

Better to not have it than to have it improperly mounted.