You could try the French South West, and Mediterranean coast. It is very, very flat, warm, lots of hotels and gites. Or, you could try one of the river routes, such as the Danube, Loire or Rhine. or the German Romantic Road. They are all geared up for cyclists, and there are cycle-friendly hotels with storage sheds. As people have said, there are lots of quiet roads that run the same way as the Canal du Midi. There is also Pomerania.
I remember Lieselotte, from Furth, and La Richelle has une tour.