Hi Ian,
We are doing a similar trip leaving on April 19th, but decided that it would be easier to go via Portsmouth - Bilbao Ferry, as it worked out not much more than driving down through France. It also means we arrive fresh and ready to go, with an extra 2 days in Spain as a bonus, have you considered this option?
I also asked a question on Toll charges in Spain and received some excellent information from Ray S & JayDug, if you look back you will be able to find the post, entitled Spanish Toll Charges 4months and 3 days ago, so some time in November. Some of the routes are over the Pyrenees so may be what your are looking for.
Be aware that if your rig is longer than 12metres then you will have to put 2 Marker Boards in Red/Yellow Chevrons on the back of the van. Look on ww.freightproducts.co.uk if you need them.
Our plan is to do the central area's of Spain including, Pamplona, Salamanca, Madrid, Valencia and Barcelona before working our way back up through France via Millau to Calais in June.
Carcasonne is well worth the visit, use the Camping De La Cite if you can, as you can walk in to the town within 10 minutes.
Just had a thought are you a member of ACSI ? if not consider joining as you will recoup the cost with 3 nights pitching, especally as you are going our of season. Just put ACSI on the net and it will bring it up, the cost is about £12-00 or less the two books are woprth the money alone as they give you over 3,000 sites all over Europe.
Hope your trip goes well, I am sure you will get some more relevant advice, and when you do I will be looking to read it myself.
Regards, George