So if I'm in, say, Ljubljana, and I find a trip starting there that I want to take, and I want a sleeper to Munich and then I want to switch in Munich to a TGV that requires a seat resv plus a supplement, how do I actually DO all that from a computer in a hostel in Slovenia!?
The problem is that it's not always even possible to get a seat reservation online, for instance on DB website they don't sell international seat reservations separately, but only with ticket. That is why the simplest option will be just to visit international ticket counter in Ljubljana, it will be much less hassle. First of all it's basically the only place where you can purchase your sleeper reservation (no supplement needed in this case), plus, as far as we know, in Ljubljana they do have access to international reservation systems, thus it won't be problem for them to sell you reservations for Germany, France etc (price should be the same).
Also you might acknowledge the fact that it's rather easy to save you 10 EUR and avoid reservation for Munich-Strasbourg journey. It's really easy to accomplish, since Strasbourg is just across the river from German Kehl, while till Kehl you don't need any reservations. From Kehl you can either use city transport or local train (also no reservation needed). Thus you basically save money just by making one train change
The only obstacle is that TGV train doesn't stop in Kehl, thus if you really up to using TGV from Munich, then you'll need to get off in Karlsruhe (last TGV stop in Germany) and change there for a local train towards the border.
With Strasbourg-Valence and Valence-Narbonne segments you might not even need to prebook any reservations in advance, both connections aren't the busiest one + April is kind of a low season, thus there shouldn't be any problem with buying reservations (again no need for supplement) already in France
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