Regarding overnight parking at the ferry port at Portsmouth, Brittany Ferries website advises as follows
For early morning sailings, you can park your caravan or motorhome in the check-in lanes after midnight. Your vehicle is parked at your own risk and should not be left unattended. No port facilities are open overnight.
How much latitude you should expect over the ‘after midnight’ restriction may depend on when you arrive and how busy the ferry-port is. I think the latest sailing in 2020 will be at 23:30 to Le Havre, and from that time on the check-in lanes should be available for overnight parking and the check-in booths should be closed until the following morning. If it is quiet and there’s room to do so, Brittany Ferries are also normally prepared to allow parking on the periphery of the embarkation area.
Your enquiry comes up quite often on motorhome forums and it’s commonly suggested that, rather than overnight at the ferry-port itself, it’s more pleasant to stay on the large car-parks at Port Solent about 10 minutes drive from the port. This is an attractive proposition for motorhomes and I normally stay there for an hour or so before taking an afternon sailing to Ouistreham. But parking there might be trickier for anyone towing an 8-metre twin-axle caravan.
(Port Solent car-parks GPS coordinates below)
50°50'26.2"N 1°05'43.0"W
50.840613, -1.095267