Shower Pan Flex

[Andy Smith]

Hi Folks!

This is potentially paranoid rambling, but I wanted to check and see if other people had seen this issue and a potential repair. I noticed today that when stepping on our shower, there appeared to be a small amount of flex in the floor. This is not near the drain, but midway between the edge and the center of the pan. The pan is fiberglass, and appears to be well-supported near the drain. There is no access panel in the bathroom, and the only opening from below that I can find is where the drain line runs to the grey tank (and only has an inch or so opening around that so I can't really see inside to check.

Is there any other way to check for support under the rest of the floor, or bulk up the existing support without taking out the shower or drilling another access hole in the floor? I had read about adding shims or expanding foam, but I think I would need to be able to see where to add them. The flex is super minor, and only when stepping on one spot.