Years ago you could camp right on the lake in primitive walk in spots, now that is not allowed. There is a small campground with just a few spots down the road a quarter mile but it's rare to find an empty spot there. The lake has grown in popularity over the years and if you want to secure a day spot on the lake's edge plan on getting there first thing in the morning. People do also disperse camp about another mile down the road in a gravel pull out area. Castle Lake is easy to get to via Castle Lake road which is paved the entire way.
The smaller sites at Juniper Lake are first come first serve, unlike the group sites that can be reserved. Over the last few years the popularity of Juniper Lake has grown and it is difficult to find a spot on weekends, but early to mid week there's a pretty good chance you can find a spot. If you're lucky enough to get a spot on the lake you'll have one of the most beautiful camping spots in Northern California with an epic view of Mt. Lassen. There are a couple of good day hikes nearby as well. Typically camping here is tent camping, but if you have a smaller trailer like a tent trailer or Rpod you might be able to get it in. There are no RV hook ups.
Part of Butt Valley Reservoir, this is the upper (and nicer) campground on the lake. Pave roads are great for kids for bike riding, scooters and skateboards. The campsites are spacious and the lake has great fishing. One of our favorite spots and it's very easy to get to.
Most people flock to the nearby Lake Almanor or Butt Valley campgrounds, so Yellow Creek Campground is a nice place to go to escape bigger and busier spots. Nestled in the pines and bordering the Humbug valley, this campground has some of the best views in Northern CA. It's off the beaten path a bit. About an 8 mile trip down a gravel road but very well maintained and easy to get a smaller RV back in there if you like. There's a natural soda spring on the entrance road where the local indigenous Maidu used to gather and drink for healing. https://www.outdoorproject.com/united-states/california/soda-springs-historic-site