Onion Valley CG is a great place to stay before Whitney. I stayed there Tuesday night 7/5 before we summited Whitney yesterday the 7th. I was heading from the south and the extra drive of 40 minutes from Lone Pine to Onion Valley CG was well worth it.
Aspens, various pines, waterfalls, quiet, very clean campground and a host, Bill, who can tell you details about any of the surrounding trails. I hiked for about 4 hours out of Onion Valley the day before doing Whitney in an effort to aclimatize better. Switchbacks out of the CG up to lakes. Lots of good day hikes.