Closest to the park entrance along Highway 89 you’ll find the pleasant and serene Manzanita Lake Campground (179 sites, May–Sept., reservations recommended, $14). By far the largest campground in Lassen [1], Manzanita Lake has a full slate of amenities, including flush toilets, showers, potable running water, fire rings or pits, picnic tables in all campsites, and an RV dump station. Trailers and campers up to 35 feet are allowed at this campground.
Five miles south of Manzanita Lake, the more primitive and less expensive Crags Campground (45 sites, June–Sept., $8) offers a more out-in-the-woods style of camping. Crags does have potable running water and pit toilets, but no showers. Each site has its own table and fire pit.
Farther along Highway 89 at Summit Lake [2], the Summit Lake North and South Campgrounds (92 sites, June–Sept., $12–14) are among the most popular in the whole park, so reservations are recommended. Visitors can swim in Summit Lake, easily accessing its banks from campsites and paths. These two developed campgrounds have flush toilets, fire pits, and tables, but to be safe bring your own potable water, enough for the length of your trip for drinking and dishwashing.
At 6,650 feet altitude, the Summit Lake [2] campgrounds are among the highest in Lassen National Park [1]. Be sure to take it easy setting up camp on your first day so that you can get used to the thinner air.
Links:
[1] http://www.moon.com/destinations/california/lake-tahoe-and-the-northern-sierra/lassen-volcanic-national-park
[2] http://www.moon.com/destinations/california/lake-tahoe-and-the-northern-sierra/lassen-volcanic-national-park/sights/summit-lake