My truck is a little differant than yours, but not that much.

   The plastic tanks are a little differant than the old brass and alum
ones.  The petcock on my truck is located on the drivers side, at the
bottom of the tank, on the side (not the rear) of the tank.
   To the rear of the petcock you'll probably see a small rubber hose
in a U shape.  Pull the top of the U off of the stud, and aim this tube
in to a bucket.
   The petcock doesn't look like a petcock at all.  More like a dial.
Pull it straight out with a pair of pliers, and squirt a little
penetrating oil in there (WD-40 is not a penetrating oil).  After a few
minutes, turn the dial counterclockwise 190*.  Water should flow out of
the hose and in to the bucket.