May not be far enough south for you, but have you considered the Marne Canal path and surrounding area through Champagne region? The Marne path is long and wide, also flat and with lots of nice French villages and towns along the way. You can also branch off into the vineyards etc as and when you please. We did a week there last year staying in a vineyard Gite and either riding out from there or driving the bike to a different section of the canal and riding further.

Thanks Garry, I will explore that idea.

They reckon that proper Mediterranean weather starts from Mâcon and south from there. Last year we followed the Rhône from Genève to Avignon. Most of the paths were very wide, due to the requirements of vehicle access. Depending on the time of year it can be quite busy. This year we followed the Saône from Chalon-sur-Saône to Mâcon. Wide paths/roadway access as before but quieter than the Rhône. If you want to try out the Burgundy area, there are some Voie Verts on disused railway lines, which are also wide, although there was a very challenging switchback in one of the towns, which we just managed. I doubt your tandem trike would fit, but it could be easily avoided! 

Many thanks, Alan - will look into those ideas.