Look up Slapton Campsites, depends what you want, do you want great facilities or cheap as chips. We stayed in a great, cheap one near slapton village, 10 min walk to visit, 20 mins back (downhill then up!), it had a tiolet, shower cubicles etc, but not much else. Or there are two specific ones the other side of Slapton, nearer the sea, looked great, are part of the camping/caravanning club and as such a bit more expensive (a lot more?).
Are you taking children? If so, doesnt matter when really, if not, best to go when no school hols. Cheaper/less busy.
If you want absolute basic, I know some great spots on Dartmoor for absolutely nothing...
Littlelapin - you and your yurts, you are obsessed woman! Have you actually stayed in one?