..stay out of the big towns if you can...nightmare navigation etc.
The obvious routes - apart from Paris and Lyon there are no big towns that pose any difficulty - Its easy to get into the centre of Paris Marginally more difficult to get out out south - Lyon is dead easy to get in and there is a great big River you can follow to get out - anywhere else is not worth worrying about
its a Fallacy that towns are difficult - towns are often some of the most interesting pieces of the ride - Ive done France North to South eight times and Ive not bought a single stadt plan for any town