I can't help with med specifics, but we trailer to a lake near Bordeaux. The lake Police ask for insurance, boat reg and ICC plus they have a list of safety items - all of which they never check, simply pocketing the seasonal boat license fee!
UK Police at the tunnel showed interest last year - they wanted boat reg and passport details. I assume they suspected people smuggling back into the UK.
In terms of the journey, we are usually in a rush and travel on autoroutes so we have a toll tag - great feeling passing the queuing French motorists who flew by a few mins earlier. Viamichelin is good for route planning and forecasting the toll costs.
I'd also think about your stop-overs; book them if peak times and check trailer access/parking. Avoid driving on Saturdays!
We've been using the tunnel for the last ten years, but we're trying the ferry this year - Portsmouth / Saint-Malo overnight - to reduce the monotony of driving on the Autoroute.
I'd like to visit the bay of Roses in Spain, looks lovely - I'd consider getting the Bibao/Santander ferry to reduce the driving time. Corsica, Portugal or Morbihan all look amazing...