We just spent the night of 5/23 at Ravine campground which worked well. We showed up around 7pm on 5/23 and were able to grab 2 sites, there were about 5 available sites when we showed up.
To get to Ravine campground, just park in the hiker overflow lot directly in between the Whitney Family Campground and the Whitney Portal Campground. from there, walk down the trail to the East a few hundred feet. This walk is probably the most inconventient thing about the Ravine campground but it worked for us because we only stayed one night before our dayhike.
