We are hoping to explore these areas in a campervan in May 2018.We have just over a month.
Are we travelling too far?
We are leaving from France/ Italy border and would like to see Venice at least in Italy but aside from that are flexible.
We would love some advice on best places to stay and see in a campervan and advice on timing as we really have no idea coming from New Zealand.

There are dozens of possible itineraries between
Chamonix or Montmelian or Briancon or Barcelonette or Tenda or Sospello or Menton on the one hand and
Koper or Rijeka or Delnice or Zagreb or Varazdin on the other hand.
May be you tell us what are your interests, what kind of places you like to visit.
And why Switzerland (which is obviously not along the way)?

I loved Switzerland on my last visit 30 years ago but if similar scenery then can miss.We want to experience smaller villages rather than typical tourist spots.
Will have bikes too so cycling where we can .Cafes, wineries, swimming beaches ?? Just getting a feel of the culture without rushing through.

If you don't want to rush, note that Croatia alone can easily take a month at a very fast, hard pace.

You would, I think, do well to consult some guidebooks.

Here is a 42 Km bicycle ride from San Candido, in the Sud Tyrol region of Italy, to Lienz, Austria, where you can ride a train back to San Candido with your bicycle. All small towns, off the beaten path.

https://www.italybeyondtheobvious.co...-lienz-by-bike