Hi, have a look at Kenneggy Cove Holiday Park near Penzance - it's not adult only, but it is only small and very peaceful. The showers and toilets are excellent, the coast path is a 5 minute walk from the site, and there is a gorgeous sandy beach which is only accessible from the path, no road, so it is deserted even in the height of summer - plus you can take dogs on it in high season too. A little gem of a campsite imho
Amazing family weekend with old steam engines, classic car displays, market stalls, and full catering and bar. And camping on site - Save £25 by booking in advance.