East Fork Campground is off Rock Creek road, 3 miles below Rock Creek Lake, and about 4 miles below the Mosquito Flat trail head, from which you can access the beautiful Little Lakes Valley, and beyond into the high Sierra backcountry.  
It is a huge campground with 133 sites, and stretches for a mile or so along Rock Creek.  Some sites are in tall pines with good cover, and some are in scrubby aspens, or open meadow with very little cover.  Some sites are well spaced, and some are right on top of each other.  A few sites are right on the creek.  About half of the sites are on a reservation system, and about half are first-come first-served.  In my opinion, most of the best sites are not reservable.   All sites have a sturdy old-school picnic table, and large bear box.  Some sites are suitable for a small RV, but most don't have the space for large vehicles.  The team of camp hosts were all very nice and were doing a great job keeping things clean and organized.  One of them shared with me that the campground was designed in the 40's, before large RV's existed.  Today's rolling condos just don't fit.  (Another plus in my book)
We camped here for 10 days in late June - early July, basically the peak camping window in the Sierras and the campground was only full on July 3 & 4.  Otherwise it was about 1/2 full.  Totally possible to get a site without reserving ahead. 
We drove in on a Thursday morning, with no reservations, after checking out other nearby campgrounds (Rock Creek Lake, Upper and Lower Pine Grove).  Open sites were plentiful, and we happened to snag what is, I think, the best site in the whole place.  Huge, right on the creek, and no close neighbors, yet not terribly far from the restrooms.  SCORE!  The sound of the rushing creek muffled all undesirable campground sounds. 

Bathroom facilities offer flush toilets and running water, a very nice surprise.  
This is a great base camp from which to explore the Eastern Sierras and Owens River Valley.