Did 8 weeks two years ago, absolutely loved it. We did holland, Germany, Austria, Slovenia, Croatia, sailed to Italy, France , Northern Spain and sailed back from there. That’s probably too much ground to comfortably cover in 6 weeks unless you enjoy the driving bit – we never travelled on consecutive days, and the longest drive in a day was 220 miles.
Of those I would strongly recommend going to the Basque Country, lovely scenery, beaches, food, drink etc….. And you. can sail back from Santander or Bilboa rather than trudge up through France. We could have done with more time in Austria too, we just passed through, but have been back twice since.
It was all great, stayed at a few aires, but tbh a lot of the campsites in Europe were good value anyway. We took bikes as they were great for getting around rather than driving the van all the time.