sluetke,
I assume you're referring to the RS Adriatic tour? I took that tour in September, so travelled the same routes you're planning.
You might consider flying inbound to Munich as there are a few direct trains from there to Ljubljana every day (travel time about 6 hours), and others with one change. I always like to arrive a few days prior to the tour to get over jet lag, and Munich would be a good place to spend a few days.
Another option from Munich to Ljubljana would be to take the GoOpti Shuttle. I looked at that but felt the train was a simpler and more direct method.
Getting out of Dubrovnik at the end of the tour can be a bit challenging (especially as there are no rail connections there). The method I used was a flight with Croatia Air to Zürich and I headed into Italy after the tour (that was actually two flights - first to Zagreb and then on to Zürich). You should be able to find a good choice of flights from Zürich to Detroit. Zürich is a very easy airport to use, as it's only about 10 minutes by train from the city.