The Lower Thames Crossing will likely be open towards the end of this decade. That will give a crossing from Gravesend to TIlbury as well as an upgraded road network around there. This will mean any freight headed north won't even need to go via the Dartford crossing.
Lets not forget Tilbury and Thamesport are both busy sea freight ports out in the Estuary and although they are not as far as Manston it isn't much by road. Seeing how freight moves around the country via distribution centres only to sometimes end up not far from where it started I don't see the issue. There is also a new station nearby and the possibility of rail freight should it prove to be a success.
With more imports/exports to places other than the EU post Brexit I think it could be a success and hope it is.