Train or car?

There are 4 adults and we would make a few stop along the way, but plan to do it in a day.

If you have stops in mind, then driving is probably what you want. If you were renting a car in both regions, any way, then I'd keep the same car.

If you want to use train, the departure I'd choose leaves Avignon TGV station at 9:14 (allow time to get there from the Centre station) and runs directly to St. Pierre-des-Corps. There can also be a later afternoon departure from Avignon Centre station to Nimes, and then connect there to St. Pierre-des-Corps. Both are running most days in July and August but shorter in the fall, such as Sat-Mon. Avoid routes through Paris, likely to cost more and they require changing stations in Paris.

In a day, you have time for one stop rather than a few. Still, with four people driving can be economical, so you'll have to make the calculations. Reckon on 10 cents per km for the highway tolls, and same for gas.