I did Gibralta to Santandar via Salamanca no tolls decent fast roads one hit 11hrs no worries, Fueled up in Gib and again about 45min from Santandar.
Having done over the years Caen to Bordeaux many time this is approx 8-9hrs 1 tank so you need to be stopping somewhere near the boarder to smash it in 2 days (2 full on days) it will be into the 3rd tank of fuel. And yes dinner and drinks on the boat are very expensive. But a nice way to travel in my mind but very expensive.Ill do it again If everything is under control you are not going fast enough.

MY 16 Discovery 4 Commercial Workshop and Escort Vehicle
MY 12 Full Fat TDV8
1952 Series 1 80" 3ltr 6cyl