Mazama Village Campground is located seven miles from the rim of Crater Lake. The campground is mostly forested and each site has a picnic table, fire ring, and bear locker. 75% of the sites are are reservable in advance and the rest are available on a first-come, first-served basis.
There are a ton of tent and RV sites available between June to October (really depends on the weather).
Make sure you book ahead of time though, as they do go fast.