I lived in Milan for 3 years, apart from the cathedral and "The last Supper" I would give it a miss.
Given you are probably going to get ripped off in Verona and Venice, you won't have enough left for Milano culo del mondo
There is something in almost every Italian city but you really need to know it's there and go for it: not too difficult, most towns have websites.
Good tip - don't take your car into any Italian cities. There are some here who will tell you that Milano is the worst place to drive but I'd share that accolade between Roma and Firenze (Florence). Stop/stay somewhere away from the city but on/near a railway line - Italian railways are good, fast and cheap.
Don't forget to leave a review of the French and other European campsites you have visited!
Personally I'd go to Firenze in preference to Lucca. Sienna was well worth a visit. Verona? apart fom the arena - not much there.
You won't necessarily be 'ripped off' in either Verona or Venezia. we got a really nice meal in Venezia for €15 3 courses. We saw some workmen going into a samll restaurant so followed suit - really nice meal. Of course if you want Harry's Bar you will pay Harry's prices - around €15 a coffee- but the music is good!
Do what you like doing really. At worst you will be disappointed a best you will enjoy yourself and want to go back again - hence my comment about Lucca etc