seat reservations

Since, as you've seen, RailEurope seems to be "sold out," you can try to see what's available from the German rail network, Deutsche Bahn. They should be able to reserve your entire journey for you over the phone (for a fee) and mail the reservations to you before you leave. The important part is the middle part....the night train from Metz to Augsburg...that's where you want to nail down some sleeping accommodation. DB's number from the U-S is 011-49-1805-996-633. Try to dial daytime, their time, to get an English-speaking agent...they're 6 hours AHEAD of you.
The only other option might be Euraide (www.euraide.com). They will make reservations for you at European prices...but they charge a hefty fee for the service, something like $50.
I'm hoping Lee has some other alternatives! ;)

You can book seat reservations on the CNL train, leaving Paris at 20:20 and arriving in Augsburg at 06:33, on the German Rail site. Click on "Check availability" for that train and then click on "Book only extra charge." Click on the bird's eye for "Pass offer" and then click on "Continue." Select "Day couch" and "Book selected offer." Proceed to complete the booking. You'll pay €4/reservation.

Take a regional train from Augsburg to Fuessen, leaving at 07:11 and arriving at 08:56.

It somewhat depends on the start and end of the night train. Share with us.

For an overnight train, you will DEFINITELY need reservations. As Lee notes, it depends on where your overnight journey is....let us know, and someone will come up with some alternatives.

We are leaving Paris on Tues June 29 and want to arrive in Fussen on Wed June 30 to see Neuschwanstein Castle. I realize it's probably too late to get a sleeper car or bunk bed, but would still like to be sure we have a reservation on an overnight train.