We think the best route is autoroute to Senlis, off towards Ermenonville, down towards Paris and around the N104 (Francilienne) then Nemours, Nevers, Moulins, Grannat, (N roads, then on the junction to Clermont Ferrand) then the A75 over the Millau viaduct toward Montpellier, then take the Beziers turn off and head down the (whatever it's called) to Narbonne, and at Narbonne Sud take the N road toward Perpignan and into Spain. In Spain you can choose autoroute or national road and there's not much to choose between them in terms of speed.
Cambrils Park is lovely - we've just come back!!! WE did this route both up and down and had no problems either way. On the way back we did Cambrils to La Grange Fort (just south of Clermont Ferrand, then La Grange Fort to Calais) in two days (OK long ones, but we were welcomed at Chateau du Ganspette with a lovely pitch, and a meal at 9.30 in the evening! Fantastic end to our holiday.