mounting fan/light in very tight enclosed space over shower
You're between a sheetrock and a hard place (sorry). I can't count how many times I've seen this done and done it myself. The weight won't be a problem assuming the sheetrock was fastened properly to the joists. I really don't know another way to do it unless you have a six year old that can fit in there.
 What are your plans for venting this to the outside? Do not just vent it into the attic. Good luck.