Jennycliff is a bay within the Plymouth area There is a public carpark overlooking it at the end of Jennycliff Lane.
I have been up there in a car but not tried it in a MH. You would need to examine google earth carefully to see whether suitable for you. It’s quite a way from the ferry port and you would have to navigate your way through Plymouth and it’s suburbs.
We recently came back through Plymouth late evening and parked up right by the port no problems. If you come off the ferry and right by the entrance to the ferry terminal but opposite on the left there is a patch of ground overlooking the Marina, we parked up there.