The passenger cab window when opened, 2018 Fiat Ducato, whilst driving lets in a damp smell, close it up and it goes... not there when stopoed. I have checked the drain vents at the bottom of the door, been smelling everywhere but just cannot trace the smell when stationery.

Any ideas on where to check and how to get rid of the smell.

Possibly airflow/turbulence is bringing the smell of the drain vent from the roof of the trailer forward to your vehicle while driving and when stopped it is naturally flowing upward, hence no smell when stopped?

Have you ever added anything to either your black or grey water tanks?   We use a product called "Happy Camper" and purchased it at Walmart.