We stayed at Joshua Tree for three days in April 2017. The Blackrock campground provides easy access to the most interesting portion of the park, in my opinion. We stayed in site #45 and it was very nice, located on the side of a hill (slight) and close to a couple of trails. The iconic trees dot the campground. There is water available but no hookups. Cost was a very reasonable $20 a night. We also reserved in advance.
We stayed two nights at Gambrill State Park just outside of Frederick, Maryland. Our site was #20 with 30 amp power and water, flat landing pad, fire ring and couple of other touches. We paid under $30 a night in August of 2017. There are not many sites with power. We liked our site but 22 may have been a bit larger. It was screened and rather private. There were helpful camp hosts. We did miss the turn off into the park, it was dark. I would not recommend missing that turn as you go up, way up and then need to turn around on a narrow two-lane road in the dark. But if you have Josh the ranger on duty, you will be fine. Nice and super helpful guy. Easy access to historic areas and even DC is just 45 miles away.