You could try the RAC or the AA web sites as they have route planners on them.

Using the RAC planner I tried a route from Calais to Benidorm and it gave directions and distance in miles and kilometres. 1118.4 miles 1799.8 km You can also do a route flight which allows you to follow the route.
RAC route planner remember to untick UK only.

AA does the same but no map 1816 kms 1128mls
AA route planner europe

Try posting on the ex-pats forum. There are probably more people there who have driven down with vanloads of gear. I would think the tolls are the most important thing to think about. That and somewhere safe to stop for a rest!

Hi Bluechip if you look at this thread, there is a route given by madcarravanner towards the end of the thread which may be of some help. This is off the Caravan forum.

http://www.holidaytruths.co.uk/viewtopic.php?t=87178

Best Regards - Taggy