mknapp,
Are you absolutely committed to using the train on that route, or would you consider other options? A few thoughts.....
- Train - that's going to be a LONG and probably tiring travel day, and probably a minimum of about 12-14 hours, with numerous changes. I looked at a couple of different routes, and one of the most promising was train to Lyon (about an hour), then from there to Milano Porta Garibaldi (as short as 5H:15M with no changes). There's a change at Chambery and the final leg is via TGV high speed, which is the best option. At Milano P.G., you can connect with the Italo high speed (up to 300 kmH) to Rome (about 3 hours). NOTE that you'll probably arrive at Roma Tiburtina rather than the larger Roma Termini, but it's not hard to get from there to other parts of the city. If you really want to arrive at Roma Termini, check Trenitalia high speed routes from Milano. The best solution is to connect with high speed trains as soon in the journey as possible. Note that reservations are compulsory on the high speed trains, and those will be provided with your tickets.
- Train & Air - this will be much quicker and more efficient, and a better use of your valuable holiday time. You'll first have to travel from Avignon by train to Vitrolles Marseilles Aeroport (~1H:15 depending on which train you use) and then take a short Bus ride to the Marseilles-Provence Airport. From there EasyJet offers daily flights from MRS to FCO (1H:20M flight time), with prices currently listed at €41.46 PP (plus a few fees of course).
If booking with budget airlines, a few points to keep in mind......
- Tickets tend to increase in price as the seats fill (sliding scale), so those who purchase early get the best deals.
- The cheapest tickets are usually non-refundable and non-changeable, so choose carefully!
- Be sure to read the Terms & Conditions carefully, especially related to luggage. Some budgets only allow ONE carry-on item per passenger, and they mean ONE. Also, carry-ons may sometimes be put through a sizing frame to ensure they're the approved size.
As the others have mentioned, DON'T use Rail Europe for researching anything. Use either bahn.de, Trenitalia or SNCF. If you decide on the longer rail option, you can purchase tickets online at Capitaine Train, Italia Rail or other sites, as they are often more "user friendly" than those of the rail operators.
Good luck with your planning!