Not sure whether you mean a camper or motorhome (some people don't know the difference). We've done Dartmoor, Bodmin and Exmoor for free. There are loads of car parks and view points that no-one uses late afternoon or evening. I can't tell you a place because we never note them just go with the flow. The worst that can happen is a Ranger or bobby asks you to move on. It has NEVER happened to us. Well once on the Sandringham when the Ranger knocked the next morning and said, "Would you move on after breakfast please, her Royal Highness does not allow overnight camping"!
In a motorhome it's a bit easier just find a public loo every few days to empty the loo and when you fill up with fuel you'll find most garages especially the smaller franchises are happy to let you have water.
If it's a camper (we used to have a MH and now a camper), unless you have a loo, it'll be cat scrapes.
I can't see the point in using the camper during the day and then going to a site to park up. I can do that in beautiful scenery elsewhere. Waking up to a mist with ponies walking through it is just fantastic.