France Motorail will get to the France/Spain border overnight (Fridays usually). You can then make your way into Spain. As far as I know, there's not a cross border motorail route.
I use
these guys for my trips to Nice (then back through the Alps) but they also go to Narbonne.
This site says that there's no Motorail in Spain at all...
(Can somebody move this to Eurotour please - that's where it should be.....)
.