HI, now I would say there is a choice of two good routes,
1./ Milan, Turin on the A4 then into France, A43, A49 to Valence, then A7 down.
2./The other route would be the A7 down to the Italian coast, then the E80 through Ventimiglia into France, then the A8 onto Nice, Frejus, Aix and on to Nimes
Personally, at this time of year I would give the coast road a miss. You will get lots of holiday makers heading south on the A7 in France, but it will be a doddle compared to the 2nd option
There is a 3rd option but not sure if trucks are allowed to use it anymore. Been a while since <i went that way.
Which is head towards the Frejus tunnel (opt2) then at Ouix head towards Montgenevre on the SS24, down to Briancon and Gap, then pick up the A8 further down. It is hard work but worth it for the views