2002 Yukon wont start

2002 yukon xl 78000 miles. Never any problems. Did not start on first try last week. Parked it for 2 days, now it will only turn over. Drained the battery trying to start it, and had to buy a new one. (Wuldnt hold charge) still only turns over. Been told to change crankshaft position sensor, but cant find it in engine. Any idea? Or should I be looking at fuel sending unit, fuel pump or starter? Thanks