We will be arriving from Barcelona by TGV and then onto Aix by TER. Are these trains located in the same station and how much time is needed to make the change?
https://en.voyages-sncf.com/en/
This site, avoiding the dread RailEurope bump, shows one train daily with no changes from Barcelona to Aix en Provence.
It also shows three trains that require a change in Valence. These rides take at least an hour longer, explained by the length of the transfer. The new station services only TGV so locating your platform will be simple.
To find these schedules, be sure to switch off the box for direct trains only. The rolling stock will be all TGV, no TER district trains, and with seat reservations which you may be able to print at home.