Battery lifetime about a fifth what it should be
What makes you think you're actually draining the batteries? 

The batteries are not going to provide you with 4.5V all through the 1200 mAh of charge. The voltage is going to drop. According to your measurement(s) the battery pack drops to below (I assume) ~3.3V after two days of discharge. At which point your buck doesn't have anything to buck and the rest of your system browns out.

Try using a buck/boost to get more out of your battery pack.