Belle Fourche Dam & Reservoir

Great find! GPS took us to the wrong place so I wanted to give more detailed instructions.
Follow signs for Rocky Point Recreation Area (this is the pay campground).
From Hwy 212 take left onto Fisherman's Road. (Google Maps said Belle Fourche Dam and Reservoir is 0.6 miles on the left. It is a large dirt "parking lot" that says "no camping")
Instead, once you get on Fisherman's Road, go down 1.9 miles. You will see a shack on the right, make a left right onto Susie Peak road, before the stop sign at that intersection. You will see the brown sign for Belle Fourche Dam and Reservoir. The road is all rocks and gravel and very dusty!

Go down about 3.7 miles and you will come to the first loop (Middle Point Road) on your right. It's a pretty large loop, but not much shade.
You can continue 0.8 miles to Loop 6 on your left.
Further 0.2 miles is Gadens Point Road / Loop 3 on your right. We stayed there and it has more shady spots.
Keep going up Susie Peak Road for Loops 4 & 5 for more options. Loops 1 & 2 are all the way at the end and have even more shade (which is probably why it seems to attract longer stay residents.

All loops have vault toilets. Great place, very quiet (and we were there on Labor Day weekend).

Very clean too!
It's an amazing find!

Specific sites are along the water but we parked out in the open grasslands at a spot before reaching the formal entry to the lake area. Great spot, water views, birds, even a couple of deer. There is a pay area with reservations if you want that, but plenty of free camp areas too. Dump and water available just past the pay area entry kiosk. Don't know if this is free. Also a very nice dump and water at the visitors center in Belle Fourche.

We stayed a week and had 6 days of crazy wind mixed with extreme calm. I loved it but plan for 50-60mph wind at times. The area is huge with several locations to choose from around the reservoir that accommodate all size rigs. You can take multiple loops to find an open spot and some are very remote. You can camp in any designated camping site. It was very busy when we arrived on the weekend but it cleared out during the week.

We had both AT&T and T-Mobile service with around 6 to 10Mpbs download each. The town of Belle Fourche 6 miles away has some interesting sites to see and we enjoyed Spearfish nearby too. Only 35 mins to Deadwood.

There is garbage bins located all over and a dump station with potable water near the entrance.

LAKE SIDE FOR FREE! This is water sport heaven or just lake side relaxing. There are ample sites for all sizes of rigs around this reservoir. Each site is clearly designated by a fire ring, so there is no tramping willy nilly onto the grass lands. The sites are spaced far apart for spreading out or just plane solitude. You can choose from total sun spots to varied shade. If it’s been raining there will be water in low spots, so scout your site before hand.
The res is about a half hour from Belle Fourche which has any supplies you may need and the towns people are great. I just love small towns!
At the entrance to the reserve is also the entrance to the pay park Rocky Point Rec Area where there is potable water to had.

Drive straight past the lot that the GPS coordinates take you to. Follow Fisherman’s Road all the way to the gate house for Rocky Point and turn left on Suzy Peak Road. Follow Suzy Peak for a mile or two and you will start to see spots along the water. We camped all the way out past the last bathroom in a spot under the trees with a rope swing. The direct drive into our site was pretty muddy so we drove over the hill from the next site down to get in without issues.

I turned north (left) on Susie Peak Rd immediately before the ranger station then drove 1.8 mi/2.9 km on the well-groomed gravel road before turning right onto the first loop. There are many sites that are easily visible in an aerial view.

The one night stay was quiet, and I parked on the grass near the water.

It rained for hours after I arrived, and the grassy areas become saturated with water, and extremely soft and slippery. My 4WD truck could barely move itself, and I could not move my trailer without the help of a local, his 4WD truck, and a tow strap.

The local told me that if the ground gets wet there, make sure that there's nothing but packed gravel between your tires and the road, or just don't try to move. Apart from that, great boondocking area!

The GPS coordinates listed here take you to a small parking lot with a toilet, dumpster, and an information sign about the Belle Fourche Dam. Instead of continuing up the main road, we took a dirt road directly across the street for about half a mile and then pulled off on the side - there are multiple mown-down grass areas there where people have parked before.

We got in at dark and upon getting out of the truck, we promptly got eaten alive by mosquitoes. The dip from the dirt road to the grass pull-off was also a steep drop off, and it was close with the bedrails of our truck and the 5th wheel. Be careful if you're pulling a trailer into this.

There is a large gravel turnaround near where we pulled off. A Class A was parked there in a way where no one else could use that space, but if oriented correctly, a couple Class As could comfortably park there.

Our cell signals, both AT&T and Verizon, were not good. I would get a sporadic bar of 4g, but it would be short-lived and downgrade to 1x. Not useable signals unboosted.

In summary, fine and quiet place to park for the night (as long as you stay indoors to avoid the mosquito hordes). But I would not seek this particular area to camp at for an extended stay. That being said, we didn't seek out the spots that the other reviewers seemed to have found, so maybe there's a much better camping area that we didn't see.

Couldn't be much easier to find and set up so good for even just a night or two. Avoid camping near the main road because it gets very dusty and there's enough traffic to make it bothersome. Not a lot of people here even during our summer weekend and it's real quite at night. Bugs were not bad. Dump and potable water at the fee campground, but it's cheap ($4?). The visitor center in Belle Douche has a nice museum worth visiting.

I spent two weeks here, and enjoyed it very much. Easy access good cell service with Verizon, and deserted for the most part probably due to the lake water level being so low.
There was a pit toilet and dumpster within a mile of my spot, and I had no trouble getting level with my 29' MH. The gravel roads are fine except for a few sections with washboards, and I was lucky enough to pick a spot where the dust from the road blew away from my site. It's not far to town, and within a reasonable distance to the area attractions.