Wanting to camp here in late July I took a drive to check out the campground prior to the trip. I did find that most of the campground is on dirt driveway. The sites that aren't on the shoreline don't have grass like those on the shore. They are mostly weeds and dirt. The campground itself seems to have almost everything except showers to make it comfortable. Pit toilets are on site. There are a couple areas with a hydrant for fresh water. The sites closest to the beach swimming area seem to be the absolute best overall. Bring a camp shower for a better experience.
When i made the visit in mid June it was very very hot. I think the temperature was around 108F. I checked the farmers almanac for late July and the temperature averages from 96 to 110. Not my cup of tea. Although the lake was very nice and I'm sure the water will feel great in the summer, sleeping during those high temps wouldn't be as great.
At this point, we had finished the John Muir Trail and just needed a cheap place to stay. Diaz lake fit the ticket - had outdoor showers and everything. Was really loud at night, lots of drunk folks. People seemed to use this place for power boating and partying, which isn't really my cup of tea. Otherwise, the amenities were nice, and there were plenty of nice camp sites to choose from. Layout was a little confusing, so grab a map ;)
Diaz Lake Campground is right off HWY 395, has a small lake, picnic table, bathroom, and parking. Pros: easy to access, more like and in and out area. Accept pets. Flat surface. Lake can be a nice spot to hang out and watch the surroundings. Cons: too exposed to the highway an cars. Can be crowded and noisy. Relatively more expensive compare to other campgrounds in the area but not too much. In summary, this is a great place to go and sleep and then continue with your trip.