We did a tour of Central Spain earlier this year and had no problems with driving there, the roads were excellent and compared to the UK generally very quite. The lorry drivers do drive quickly and a little agressively but I soon got used to that. We chose to get the ferry from Portsmouth to Bilbao and it worked well for us, as it saved two days, driving, approx 900 miles and we arrived in Spain fresh. It cost us about £100-00 more than driving through France,, but this will depend on when you go. if you do this make sure your van is inside the ship, or it will be covered in salt as ours was. We then drove back through Central France to return via Calais.
Depending on your time available for the whole trip, I think it will be better to plan a route south through France, using the Peage were necessary. If your route takes you through Clermont Ferrand you then get a free motorway to Beziers, not too far from Spain.
The scenery in Spain is not a good as France, but the old towns are all very interesting with many old buildings untouched by war and the weather should be better.
Well worth doing.