The last 2X we have gone to Port aux Basque and got in late, we drove up the highway a short distance - maybe 3 to 5 miles and there is a large tourist info center. Several RV's overnighted there and in the morning we went in and got information for the Island. I think they open at 8 am. Maybe a phone call to them to confirm that overnighting is still allowed would be a good idea.
There is also a c.g. close to the ferry dock that stays open to accommodate the late ferries. Sorry I can't remember the name as we have not used it but I have seen ads for it. Possibly someone on here can provide that.