On our outward journey from Calais we always stop at the Baie de Somme Services an hour and a half from Calais on the motorway. Ok, so it is 12 euros toll but think of how much diesel it saves than going the other routes. Firstly it gives you chance to customise yourself to the other side of the road. Secondly, normally you can top up with water there free of charge saving extra weight of the drive. The toilets there are spotless and I think the showers are still free just hand your keys in as security.
There is special motor home parking but do not park where the caravans park.
Within 5 mins of leaving you are on the free dual carriageway to Rouen. W