There's a camping site in Boppard,
Camping Sonneneck and two in St. Goar,
Camping Loreleyblick and
Camping Friedenau. All are on the banks of the Rhine, so if you're lucky, you may get a pitch with a wonderful panoramic view over the river. I've stayed at Camping Loreleyblick and can't fault it, but, and this is something that applies to all camping sites on the Rhine, the valley in that section of the river is relatively narrow and main roads and railway lines follow its course on both sides, so don't expect any of them to be particularly quiet.