Found this on the Whitney Portal Store site, which mostly answers my questions:
13. Where can we camp, at or near the Portal?
You can camp for one night only at the backpacker campground which has eighteen walk-in sites (no reservation needed). A new walk in campground (Ravine) was opened in 2012 now we have 25 walk in sites
Advantage of walk in sites:
1. They are several hundred feet from the Trail Head
2. Parking at the trailhead
The Trail Head sites are not on the reservation system you do not need a reservation just come they will never be full since they do not show on the Reservation system. Most people don't know about THE WALK IN SITES, So during July and August the walk in sites are your best answer,
