Train from Paris to Arles, then Arles to Barcelona

A couple of direct services each day eliminate the need to transfer in Avignon.

TGV 6193
Dep 11:41 PARIS GARE DE LYON (France)
Arr 15:39 ARLES (France)

David, That would be great! Which website did you see those trains listed at? Are there other places to book trains that I don't know about?

Arles has a music festival this on July 10-16. Perhaps you hit some of those days? There's also a contemporary photography festival beginning on July 3, but it extends into September and seems less likely to have a major impact on demand for hotels.

I think there's an 8:43 AM TGV from Avignon to Barcelona that doesn't require a change at the border, but you'd have to start out earlier to get to Avignon in time.

I don't see how you can get a train from Gare du Nord to Arles. All the trains from Gare du Nord are heading north and northeast, as its name would imply. Trains to Arles and anywhere to the south would leave from Gare de Lyon.

Try the SNCF
https://en.voyages-sncf.com/en/

Then search Arles to Barcelona Sants.
This one I think may require transfer in Nimes for the TGV to Barcelona

But you may not be able to book using the the French Rail site.

JM,

Transfers are not a big deal, usually, on European trains.

The best, easiest resource for checking routes and timetables (but not necessarily for buying tickets) is the German rail site: https://www.bahn.com/en/view/index.shtml

It's easy to navigate and shows every train that has been scheduled. Use it to see when and where the trains go, and then SNCF or the rail site of your choice to buy the tickets.

The European trains are really wonderful, and you've picked some top-flight destinations! Have a great time.

Sncf is the French national railway company.
Book directly on their website (not an agency).
https://en.voyages-sncf.com/en/

Yes I've had problems booking on SNCF from United States. From what I remember they own RailEurope and try to steer users to that site which is more expensive.

Sam, you are correct! I typed Gare du Nord by accident somehow. I was probably reading another website at the time. ;)

We are in Arles the 4th-7th. None of the hotels I emailed mentioned why they were booked, but there is definitely something going around. I think I contacted at least ten different places.

Thanks for the suggestions! I will look into all of these. Sometimes it's just hard booking or thinking about booking so early when you don't really know how much time you want to spend some where. I'm so excited, though!

SNCF has gradually modernized its web presence and it is easier for Americans to avoid the dreaded bump to RailEurope. Try this site: https://en.voyages-sncf.com/en/
Specifying where the tickets will be picked up also appears to have disappeared (I enjoyed specifying Antarctica.) I can't comment on the difficulties some American credit-card users have had booking on-line, although the problems seemed to arise from their banks' creaky credit-card technology. www.seat61.com can suggest booking and purchase alternatives.
The link above will show you several trains with one connection in Nimes. That's where you catch the fast train, a joint operation between French and Spanish rail companies, with reservations required. The initial train from Arles will be either TER or Intercites. The former is the local train which can be pretty basic with no seat reservations. The Intercites allow seat reservations.