If you're heading towards Barcelona, do the trip through the main roads.... As you get to the South of France, roads get a lot better.
Riding through those long French avenues is great. Lots of visibility and long open (if not a little bumpy) straights.
The French like bikes a lot too.
I did a trip to Greece & Back some 12 years ago with a friend on another bike, was a lot of fun!
We didn't use Motorways at all and made it to the South of France on the first day, in 14 hours, from Dieppe.
The best road I can remember was an aptly named "D911".
Fully of chicanes and hairpins with perfectly tarmac'd roads.
We never got stopped by the Police once.
I guess you don't really cross the Alps going that way, but there's some fantasic roads there too.
If you are camping in mountainous areas, the best advice I can give, is to climb as high as you can every night, using a good road map to guide you. Pitch the tent on the side of the road (There's no cars if you get high enough) in a forest, or empty field.
You'll wake up to some of the most spectacular views you might ever see... !
It's cold up there though, so take enough warm clothing for riding especially at night. It'll be close to zero up in the colder parts.
The trip we did was one of the best and most memorable road trips I've ever been on. Definitely try to avoid Motorways unless you need to make up time. They're boring for bikes - and the scenery is much less enchanting.