Considering Rick's Provence and French Riviera itinerary by train...Fly RT or open jaw?

Fly open jaw into one and out of the other. They are a two-hour train trip apart, so no sense backtracking. Nice has one or two direct flights to the US during some months of the year; otherwise you will need to change planes in your arrival city (Paris, Amsterdam, London, Frankfurt, etc.) to get to either Nice or Marseille.

You are correct, you can't tour either the Gorges du Verdon nor the Camargue without a vehicle. You should be able to find day tours from Arles to the Camargue. My choices would be to stay in Arles in the south and Vaison-le-Roman in the north. Note that RS says you need to take van tours to see some of his suggestions while staying in Vaison-le-Roman. You may find that you do have to stay in Avignon because it is a tansportation hub, but beware of the huge theater festival July 5-26 that takes over the town.

If you don't want to stay in Marseille or Cassis, go to Aix when you arrive. You could still do day trips to both. The airport is positioned between Aix and Marseille.

Everyone is going to tell you it makes more sense to fly into one city and out the other. And it might. But you also need to look at the price and decide if that makes sense. I often find in spite of what Rick says this sometimes costs considerably more. I have an upcoming trip where find into one city and out of another was going to cost over $1000 more. That’s $2000 for two people. Obviously, we are going to backtrack on this trip.

You can do an open jaw or roundtrip, it really depends on your particular routing and time, for what works out. Another alternative is to fly into Nice and fly out of Paris. You can take a TGV from Avignon or Aix and arrive at the airport. There is a connection risk, of course (some do an overnight). Or you can reverse that route or use another portal city (often I find my flights route through AMS, GVA or FRA instead). Nice now has 7 airline routes flying from the east coast of the US (Delta, United, American, La Compagnie, Air France). These are seasonal (the earliest starting in March/April)

As others implied, it depends a bit on what flights are available to you, how you prefer to travel in relation to flights, and that times of fast trains have shifted the balance.

For myself, on return, a direct flight to the US, taking off mid or late morning is a preference.

So if I could find a direct flight back from Nice or Marseille, that is maybe where I would start, then compare the cost of Round Trip vs Open Jaw, adding in any incurred costs for getting to your return city.

Another option that is not out of the question, is that Marseille is a short 3-4 hours from Paris by fast train, meaning an open jaw Paris/Nice itinerary is possible. Land in Paris, Train to Marseille, travel, fly home direct from Nice.