Peter,
The RN85 "Route Napoleon" is is a scenically stunning route, and certainly passable with a caravan in tow. I did the journey from Cannes to Grenoble a couple of years ago towing the 'van and it's definitely one of the most memorable trips I've ever done.
As the route winds over the Alps there are lots of exciting hairpin bends and steep downhill stretches, so make sure your brakes are in good order before you go. I had to have new disc pads all round on my car when it was serviced shortly after the trip!
Take your time and enjoy the scenery. The approach to Grenoble as you pass Vizille is breathtaking.
Mike.
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