Vacuum erratic when electric fans turn on

[forestb]

So I borrowed a friend multimeter and went to start up my truck tonight and it turned over once and then would not turn over. The starter motor would just make a clicking noise. I looked at my voltmeter and it was down around 12 V. I use the multimeter that I borrowed and check the battery and it also was around 12 V. So I jumpstarted my truck and it started. I then proceeded to use the borrowed multimeter to check my distributor. Sure enough it did the same erratic reading then I moved the negative probe to the ground strap and it held steady at 14 V. I waited for the fans to turn on and it then held steady at 13.3 V. I have no idea why the battery drained that quickly in 2 days. I have not had any problems with my battery draining for a few years. The readings I am getting from my the borrowed multimeter are the same as the second one so I think that I am okay using my second multimeter.