Are you camping just one night before your day hike? If so just camp right at the Portal at a "walk in" site. Conveniently located very close to Portal Store, restroom, parking, bear lockers, and the trail head. $12
Thank you for that option. I already have reservations at Lone Pine for three night but would really love to camp closer so I can just walk to the trail. I was concerned that showing up on a Saturday morning all the "walk ins" might be taken for the weekend.
I'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.