You can park in the ferry port overnight, i've done it a few times now. Theres a toilet block and a cafe/bar that open in the evening and early in the morning. If you get there between 4.30 and 7.30 the day before the early crossing they do a check in and you can park in your designated lane - corners steadies down, fill up aquaroll with pipe from cold tap in toilet, fridge on gas, 12v system off leisure battery. My wife and I have a shower in the van, cook tea before bed for the night!
last year (end of August) we met some people we new from a campsite on the Costa Brava and we all sat outside drinking wine till late, it was a great atmosphere!