We have just been informed that the Aire shared with coaches near the Medieval citie has now been closed to Motorhomes and that there is a new Campervan Aire located near the Camping de la Cite site.  All new purpose built and €12 per night.  €2 for water.

http://www.audetourisme.com/diffusio/fr/ou-dormir/aires-de-camping-car/carcassonne/aire-de-services-pour-camping-cars_TFOACCAMLAR011V50AM3I.php

In addition and much better located there is a “motorhome friendly” carpark on the island midway between the old city and the Bastide.  Its a fixed price of €20 per night however it does not have any facilities.  Barrier operated and on hard standing so ok for those of us wanting to stay a couple of days or so and can manage without facilities during that time.  The Bastide is just across the river (accessed by a sort of clapper bridge) or the old city is back across the road bridge and up through the back gate.  Shops and restaurants are within easy walking distance.  This is certainly the best location for Carcassonne but rather expensive with little in return.