My '99 K1500 with the 5.7L has very weak vents. It wasn't an issue in the winter, it always got hot enough. Now that it's hot, not so much. I can turn the fan control to any speed, and it doesn't seem to make much of a difference. Is there a door between the fan and the vents that can get stuck semi-closed? Some air comes out, just not enough to keep cool.

 

Try pulling blower and see if coil needs cleaning, worked for me.
Good luck.
David