Hi Alistair
Snap!!! My wife and I are both teachers too!! We go away for 5 weeks to the Continent caravanning every summer.
We went to the French Alps in 2009 and to the French Pyrenees the year before. Despite hearing some tales of doom and gloom about busy sites in this peak season, we do NOT book in advance but just phone ahead on the day before or sometimes the day of arrival. In both France and Italy we have rarely had any problems getting a pitch and have in fact had some super lakeside and other pitches.
We travelled to Italy in 2006 and loved it. In 2006 we went to Lake Como and then on to the Jesolo peninsula, for Venice,and in 2007 Rome then Lake Iseo. Italy is a long way but you, like us, will have plenty of time to take your time. We reckon to do a maximum of 300 miles a day and stop at a mixture of campsites and autoroute aire de service. You can see details of our routes to Italy in the adjacent thread, which you've probably already looked at, "Calais to Italy".
Campsites we have used enroute to and in Italy are as follows:
Camping Chenefleur, France just north of Luxembourg - very near autoroute.
Camping Acasias, Cernay, just before the Swiss border
Camping Roma Sport Flash, Bracciano a rail journey from Rome
Camping Riva di San Pietro, Marone - shores of Lake Iseo
Camping La Riva - northern end of Lake Como
Camping Cavallino, near to Venice
All these sites are reviewed by me in the campsite search section of ukcampsite.co.uk - I would recommend that website as another source of information regarding Caravanning generally.
It is possible to drive from Calais/Dunkerque to the Swiss border on toll free autoroutes but one does have to buy 2, car AND caravan, vignettes for Swiss autoroutes. All Italian autostrada are toll roads in Italy, I think. The French, Swiss and Italian main roads are all good and as long as you pace yourself regarding driving distances I don't see why you shouldn't head to Italy next summer - my advice GO FOR IT.
If you have any further questions please post again and I'll try to help.
Regards
Tim