2002 LS 2 door, 5 speed, 2wd, 153,000 miles

I noticed today, while running the A/C for the first time since buying this truck, that if I moved the fan speed from 1 to 4 the engine RPMs dropped by about 100 rpm. I then started turning on the lights and other accessories and there was an even slighter drop, but nothing as dramatic. Also, as I was working on my brakes today my son was playing in my truck and kept turning on my lights and opening my doors. When I went to test the brakes the battery was dead. It was not that long, so I am wondering if these symptoms are a sign of a worn out battery, voltage regulator, or alternator.

Thoughts?

Thanks,
-Sean.