I've heard of filling voids under fiberglass tubs and shower bases by going from underneath and filling the void w/ expanding foam. Some say the best answer is to rip it out and do it again.
BTW: Not 20 minutes ago I just did the same thing for our new fiberglass tub. Looks like the mortar wasn't quite thick enough in one spot. The rest of the tub is solid besides the one area I was feeling movement. I was able to drill from underneath. First through the subfloor. Then, w/ a masonry bit I went through the mortar w/ a depth stop so I didn't go too far. (Since I knew approx how thick the mortar was.) Popped through the mortar. Shot foam in and filled it up. Curing now, we'll see how it goes.