Mt. Rushmore and Yellowstine around July 4

These type of sites are high capacity they handle large crowds very well. we were there on the 4th a couple years ago, still had room to park the couch.

Both will be crowded and busy. Go early to get parking spaces.

get your reservations NOW, or just after the 4th this year. For Yellowstone, I like Bridge Bay for it's location and it has water and a dump station and is very close to Lake Lodge (for buffet dining) and Fishing Bridge for store.
When you go to Old Faithful, go late morning, around 11am and stay for lunch or have lunch at Snow Lodge, then see the geyser.

For the Black Hills, get reservations EARLY in Custer State Park. It's a really beautiful state park, has elec hookups, water and a dump station and some very scenic drives. For campgrounds, we like Game Lodge campground and Legion Lake.

Probably the busiest vacation time of year no matter where you go. Find your spots that you want to stay and reserve early.

https://southdakotacampgroundownersassociation.org
sdcoa is partnered with State Parks. Find , reserve through this link.

I stayed at Custer / Mount Rushmore KOA over the July 4 week in 2016. I booked 2.5 months before that. I never felt like the park was too busy, it's so huge with so much to see and do I never felt crowded. A week later we were at Yellowstone and it was insanely busy...we didn't enjoy our time there very much due to the crowds.

July 4 is the unofficial beginning of the height of tourist season in YNP. Lately, with all the (usually foreign) bus tours starting in the spring, it's been quite busy, but it always is during camping season these days, so we deal with it. We need hookups but Bridge Bay is tight, so we usually camp in West Yellowstone now. (Grizzly is great; if you have kids with you who might want to blow off steam in a playground or pool, the KOA closest to town is good, though pricey - but then, everything is pricey there.) Agreed with those who say, make reservations early, both for Yellowstone and for the Black Hills. Years ago we stayed at the Rafter J Bar in the Black Hills and loved it; depends what section of that large campground you choose though.

Usually MT Rushmore has a fantastic Independence Day Celebration.