Liam,
There is a lorry service area (both sides of the motorway)….. motorhomes stop there but it’s not an aire/official stop. You’re best on this one (lets you on carriageway back to port).
https://goo.gl/maps/z5KS9Ap3z8v3Edsr8
I used it on my last trip, park down near the hedge up from the truck wash. About 10 mins takes you back to the port. Surprisingly quiet area as you’re quite a way back from the motorway and most traffic to & from the port is going the other direction.
