For sites try www.camping.it Would suggest lake garda and around the town off Peschiera del Garda. Some good non tires places for the van, IE Bella Italy or camping butterfly [my choice]
Its placement on the south east side of the lake makes it great for getting around,although the old town itself is pretty good.
Train station is 5 mins with good parking and for around 25 euro's each will have you in Venice in 2 hours.Alternative but dearer is using one of the coach deals these type of places have, around 55 euro's each but well worh it on a air con bus again 2 hours, but included boat transfer [nice 20 mins each way trip, seeing Venice. Indeed they normally offer an hours boat trip in the afternoon 20 Euro's, and its a private boat so plenty of space, which is important and the reason i mention the tour coach option, as it really is worth it. and far better value than those expensive gondoliers.and you see far more of what Venice is all about. 3 to 4 hours will easily cover the main strip of Venice, that's why i suggested staying on the lake unless you intend to do every nook a cranny of venice,otherwise this leaves you plenty of time to enjoy the lake and mountains.
For camp-sites on your journey try http://www.bvcd.de/campingplatz-deutschland/en/index.php if you go the German route try their tourist board too Internet] and get hold of there FREE campsite book,which is good.
If you intend to go the swiss route try these. http://www.switzerland.isyours.com/e/guide/by_areas.html Although I would favour the german route from durkirk [norfolkline] through Luxembourg Germany to the Brenner pass [less tolls]
What ever you do enjoy.......John