I haven't done the trip myself, however when I thought about it before, I think I estimated that taking the ferry to Spain cost double what it would to drive through France. France is lovely and I would drive it myself.
As for the solar panel, I wont bang on with my opinion again, however if you don't plan to stay in one place for more than, say, 4 days, you wont need one.
However if you plan to stay put for a week or two in one place, I agree with Spinner get the highest output one you can and make sure it is a full kit with the controller etc. You can buy them as portable suitcase types or have a fixed one fitted to the roof.
As for a bargain at the show, you will be very lucky.
I met a guy this weekend who has 400w on his motorhome roof and claimed it wasn't enough as he works off his laptop daily so he was adding 2 more panels to make it 800w! A bit excessive maybe but all depends on your power usage.
Rolling on and enjoying life on the road