Overnight stops between Carlisle and Cornwall

We are about to embark on our 1st long trip in our motorhome, travelling from Carlisle down to Cornwall and wondered if anyone had any advice re overnight stops enroute? We are leaving Carlisle at 5ish on Friday night, so would like to get 3-4hrs down the road before stopping and the hope to arrive near st Austell on Sunday. Please, any suggestions would be very gratefully received!