Here's a couple of links tp previous discussions about the walk in campsites
http://www.whitneyzone.com/wz/ubbthreads.php/ubb/showflat/Number/37962http://www.whitneyzone.com/wz/ubbthreads.php/topics/37444I've used the 1 night sites at the Portal but only mid week, never on a weekend. Each time most sites were not in use. According to one of the above threads the "Ravine" sites are for multiple nights. I don't know if this is true. I did go down to those Ravine sites. You can't drive to the campsite, you have to carry your gear from the nearby Portal Road.
Another option is the Whitney Portal campground but when I stayed there I drove from my campsite to the trailhead.
