I live in France so can't really help as to routes. But, as for overnights, I would never use an autoroute aire - especially further south. What I do is to leave the main road ( I rarely use autoroutes) and find the nearest village or small town. There, you will invariably find a town/village square where you can put your feet down with little fear of interruption. It's not like UK where someone will inevitably try to move you on! The only time the police have knocked on my van door was to ask if we liked his village and did we realise that,just around the corner was a first class restaurant! Usually, you will also find a cafe/bar and a baker nearby. I do this throughout the year when I just need somewhere to sleep. Can't see the point in paying for a site in those circumstances.
I can endorse those who warn you to be vigilant in Spain. NEVER leave your outfit unattended on autoroute carparks, and be prepared for all sorts of tricks to try to lure you away from your unit ( so that an accomplice can ransack it ). As ever, beware the Autoroute round Barcelona! A magnet for respectable looking young people in cars waving you down. Only stop for clearly marked Police vehicles.
Sounds daunting, doesn't it, but we 've been doing it for many years now and the pleasure of being there does outweigh the need to be extra vigilant.