You said Devon or Cornwall so here are a couple on the border between the two counties - Willow Valley at Bude if you want to be near the sea, or Langstone Manor at Tavistock if you want to on the very edge of Dartmoor. But there are hundreds of others !
Can see you have a caravan but perhaps a little more detail as to what kind of site ie. dogs or not, family orientated or not...
Been to Willow Valley and would recommend it - was great for our family. It was family run and I have written a review but not uploaded it. Nearly 2 years ago now!!