Are you going from Rüdesheim to St Goarshausen by boat, then back to Rüdesheim for the night, then continuing on to FRA the next day?
If so, the RMV fare from St Goarshausen to Rüdesheim is 5,60€/adult. The fare the next day, Rüdesheim to FRA, is currently 9.55€/adult, but will go up on September 11 to 13,50€/adult.
38,20€ for 2 adults for two days travel. It would be difficult to beat this with any rail pass.
But why not do this instead? After you arrive at FRA, take the train to Rüdesheim (1h16, 13,50€/adult) and spend the night at wherever you were planning to at the end of your trip. It's a short enough trip, it shouldn't be difficult even jet lagged.
Next morning, take the K-D boat, with your luggage, to St Goarshausen (1H45), then take the train to Trier (2H48, 31,-€ for both of you with a Rheinland-Pfalz-Ticket). Get the Rhein boat cruise out of the way on your way to Trier. Buy the RL-P-Ticket before you get to the K-D dock and show it when you get your boat ticket for a 20% discount.
If you don't want to take your luggage on the boat (people do it all the time), leave it at you accommodations in Rüdesheim when you check out. Take the boat trip, then come back on the train and pick it up before heading to Trier. That would add about 2½ hours to your travel time, but you could still do it for the same cost with the RL-P-Ticket, since it is a day pass.
Looking at the rest of your trip, there are Bahn Savings Fare tickets available from Vienna to Rothenburg odT in Sept for as low as 173,80 for 2P with the longest leg being on an ICE. Once you are in Europe, committing to an advance purchase ticket for a specific train should not be a problem, so Savings Price ticket would offer a good price with a low risk.
Similarly, Rothenburg odT back to FRA, again with an ICE leg, could be as low as 53,80€ for 2P.
I just picked a date in September and picked the lowest fare for a morning departure. There might be lower fares available later in the month; fares might be higher at popular times. All tickets are 2nd class.
I didn't check the prices using an Austrian Rail ticket from Vienna to the border and a Bayern-Ticket from there to Rothenburg. It could be less that way; I don't know.
There are also some very low fares from Vienna to Salzburg or Vienna to Munich on Westbahn (it doesn't look like they go to Passau, which would be closer to Nürnberg).
So, FRA to Trier with an overnight stop in Rüdesheim, continuing from St Goarshausen (or backtracking to get your luggage), would be 50,10€ for two.
Vienna to Rothenburg odT, 173,80/2P.
Rothenburg to FRA, 53,80/2P.
So you could at least do the rail part of this trip for about 278€ ($283, today). Your best rail pass deal is probably a 4 day German Rail 2nd class Twin Pass from German Rail at 371,-€ (for 2 people).