Lassen Volcanic National Park now takes reservations for some campsites. Loop A at Manzanita Lake, Lost Creek Campground, and loop B (North Summit Lake) and C and D (South Summit Lake). Loop E at South Summit Lake is tents only. Reservations can be made by calling 877-444-6777.
You say the trailhead is at 6800 feet, but this is incorrect. Both the trailhead sign and Google Earth indicate that the trailhead is just about 6600 feet. When I researched it, it appears it is about 6635 feet, making the hike an even 2600 feet to the summit.
We hiked it on Saturday. It is clearly marked everywhere, including in the visitors center that the hike is 7 miles round trip and 2600 feet up (6635 to 9235). This hike is pretty steep, but the well maintained trail makes it an easy hike. It is quite beautiful on top.