Towing a caravan from Aubenas to Castellane

I'm new to this, but would really appreciate any comments on towing a caravan from Aubenas (near Montelimar) to Castellane in the southern Alps.  Having spent a lot of time in the Alps we are aware how difficult for caravans some routes are, and wonder if any members have done this route and could give us advice on suitable roads. We would prefer motorways as much as possible, and can get to Barreme, but just wondering what the road D4085 is like, as apposed to the N202 /D985 as suggested by the club info. Any suggestions gratefully received. Thanks.

(Post moved to Overseas section, David Klyne)