Sadly, DB doesn't offer reservations anymore for French domestic trains. You can still book:

• Via Interrail after logging in (booking fee 2€ per person and train). Because of a long-standing bug, it still doesn't show 20€ seats when the 10€ seats have sold out.
• Via Belgian railways website (booking fee 4€ per order, pass cover number needed, request one here).
• Via French railways phone (press #85 for English, no booking fee)
• Via Belgian railways phone (no booking fee)

And in case you have no luck-busy easter travel period, then simply use the slower, non-RES regional trains. As we tell here about a few time daily as seemingly each&everyone wants to make the very same trips. It will also save you quite a lot of money!

And I forgot to add:

The best way to book Nîmes to Barcelona on the TGV (15€) is via DB (ticket office or by phone). But it does look like there's none available anymore on 21 April.

Nice to St. Raphael can be done without reservation in a direct TER that is just as fast as the TGV.

Hi Karl, Good to mention that for the Nimes-Barcelona can also be booked it via Interrail and reservations will be sent to you via mail. Cheers,

But note that Interrail can only sell it as paper reservation and because of a secure shipment it is quite hard to get reservation on short notice.
You promise it within atleast 9workdays but in fact the system shows at the end often a even later delivery date as the 9w workdays :)