Hi Taffy
My wife and visited Venice, with our 7.2 twin-axe caravan in July/August of 2006.
We travelled down through France, Belgium and Luxembourg on the toll free autoroutes. (We stopped at camping Chenefleur near Luxembourg on the way back on that route)We then went east along the N66 from near Nancy to Cernay where we stopped off at Camping Acasias.
We then crossed into Germany for a short distance and into Switzerland - a better route than entering through France. We had to buy the expensive annual vignettes for the car AND caravan to use the Swiss autoroutes.
We then drove through Switzerland and through the St Gotthard Tunnel and on into Italy. We stopped off at Camping La Riva on Lake Como. We then continued on to the Jesolo Peninsula north of Venice where we stayed at Camping Cavallino. This made an excellent base for visiting Venice using the ferry.
We did not, incidentally, book any of these sites, apart Camping La Riva, the rest we just phoned ahead the day before we got there - we tend to do that.
In 2007 we visited Rome - we used the same route out but because of car problems we travelled back via the Brenner Pass and Austrian and German autoroutes. The Brenner Pass is a toll road and a vignette is needed for Austrian motorways but only for the car, not the caravan, and they are available for short periods like 10 days.
I have reviewed all these sites on the Campsite search section of ukcampsite.co.uk but I can't give you the exact web link as my account with that site is playing up at the moment. If you log onto ukc and look in the campsite search section you can find my reviews there. My review for Camping Cavallino includes a lot of tips for visiting Venice. We loved Venice but it is, as you recognise, a long way.
Hope this helps.
Tim