Hi Peter
I have towed most of the Route Napolean on several different trips. It is generally OK and much pleasanter than the longer route to Eastern Provence via the Rhone Valley.
I remember well the stone arch referred to by Olly - I met a convoy of six Dutch caravans coming in the opposite direction!
It is on the north side of the Col des Leques between Castellane and Barreme.
Between Sisteron and Grenoble there are two alternative routes -the N85 (now D1085) via Gap and the N75(now D1075) via Serres and the Col de la Croix Haute.
The 'true' Route Napolean is via Gap. Beware, however, of the steep ascent out of Gap to the Col Bayard. Also, going north there is a very steep and long descent towards Vizille, just south of Grenoble. There is now a 2.7m height barrier on this descent, so most 'vanners will have to divert from La Mure onto the D529, which is scenic, but a bit twisty. (There is no restriction on the ascent going southbound)
The Sisteron to Grenoble route via the Col de Croix Haute has no severe gradients and the A51 Autoroute now extends south from Grenoble to the south side of Monestier, avoiding the previous traffic jams at Monestier at busy times.
There is also an excellent small CL type site just off this route on the east side of Aspres at St Pierrre d'Argencon - run by two English couples.
There is also a brilliant site at Luz la Croix Haute, just south of the Col de La Croix Haute. It is in a side valley well away from the main road with fantastic mountain views - Champ La Chevre.
Bonne route!
Paul