Ricardo CG at Red Rock SP has a few sites you could fit in. If you don't mind a couple of miles of dirt road, boondocking off Movie Road in the Alabama Hills west of Lone Pine is great. French Camp CG near Tom's Place (just north of Sherwin Summit) has some sites you could fit in. Old Shady Rest CG in Mammoth Village has some large sites. I believe there is still free camping at Glass Creek north of Mammoth. We've also stayed at Washoe Lake State Park north of Carson City, NV.
If you stay in Bishop or pass through near lunchtime, be sure to stop at Erick Schat's Bakery for fresh turkey sandwiches (RV parking in Bishop City Park).