As above - or if travelling on the morning ferry you can overnight at the ferry port for free, parked up round the perimeter fence, arriving after check in has closed for the overnight ferry.
We use the camping and caravanning site at Southbourne - about half way between Portsmouth and Chichester. It's about a half hour drive to Portsmouth ferries. A much nicer site than the one at Rookesbury Park. You can catch the bus outside to Portsmouth and Chichester and there's a train station a fifteen minutes walk away for trains to both cities and London.