I've heard that the "normal" liquid nails often will stain or warp the plastic enclosures...not sure what it would do to fiberglass. The instructions should indicate what to use to glue it. Liquid Nails does make a special one for tubs, showers, but really, the best is to use what the manufacturer says.
The tile needs to be cleaned off of all of the soap scum, etc. for any adhesive to hold.