The whole Hidden Valley area has more to offer than the Jumbo Rocks locale, but HV can be hard to get into because it's so popular. The smaller campgrounds, Ryan, Belle, and White Tank, are quieter and more secluded. Out of the three I prefer Ryan. It is farthest back from the main road. Because it is small, it fills up fast.
Weekends are harder times to find a campsite than weekdays.
Earlier in the day is better than later. My first choice would be to try Hidden Valley. Once you enter the campground take the first right and follow the road around. It leads to some campsites that are more secluded than the main portion of the campground. Be aware that only two campgrounds in the park have running water and flush toilets.