I hear this train go by every day as it passes through my village on the Blue Coast. It is not correct that you have to charge in Nimes to travel from Clermond Ferrand to Marseille. One train per day does not require you to change trains.
The southbound train leaves Clermont Ferrand at 12:40 and arrives Marseille St.Charles at 19:54. The designated number of this southbound train is 15957. You can buy a ticket online (www.voyages-sncf.com) and the price is €80.90 one way for first class and €53.90 second class. You can also buy a ticket at Clermont Ferrand station. In either case, make sure you specify the direct train, as you will otherwise be routed via Lyon and a TGV which is a much quicker although less interesting route.
Two recent changes to this train ("Le Cevenol") have made it an even better experience than before. Firstly, on its last leg from Miramas to Marseille, the train now normally runs along the beautiful Blue Coast route instead of the much less exciting inland route. More about the Blue Coast train: http://bit.ly/cm9BP0
Secondly, the rolling stock has now been updated and the train now usually runs with a modern diesel railcar which gives panoramic views from the raised sections at each end.