One Day en route from Frankfurt to Trier

There is a whole lot of historical stuff to see in Frankfurt, (churches, Jewish culture, museums, etc) so it depends on your interests. Also the dates, as there are a lot of festivals in the city.

What time do you land? I'm a fan of getting the initial travel all done at the same time. Half a jet-lagged day with transfer into Frankfurt and getting set up in a hotel and repacking and leaving early the next morning seems exquisitely painful compared to hopping on a fast train underneath the Frankfurt airport, looking out the window for 3 hours, and getting a less frantic start to the trip. Trier is plenty interesting for the extra time.

"...hopping on a fast train underneath the Frankfurt airport, looking out the window for 3 hours, and getting a less frantic start to the trip. Trier is plenty interesting for the extra time."

Trier is an enjoyable place and definitely worthwhile. If you can get to Trier for that first night, then on DAY 1 of your tour, you'll have almost a full day to see Trier on your own before your 4 pm tour meeting, right? So I second the idea of sleeping in Trier. What time do you land at FRA, exactly? If it's in the morning, you'll have lots of time and some good choices that day.

The easiest and most direct train trip to Trier is to take the a west-bound regional express (RE) train from FRA to Koblenz and then another RE train from Koblenz to Trier. This is possibly the most scenic train ride in Germany. So if you're just completely beat, you could just ride the trains directly to Trier. If you feel like seeing/doing something, pick something along this route so as not to add unnecessary miles to your journey. (Frankfurt proper would take you to the east, in the opposite direction.) Good options would include...

MAINZ (on the Rhine River, 25 minutes by train west of FRA. Use lockers to stow bags.) It's a city you can see on foot if you have some post-flight energy.

BINGEN + A RHINE CRUISE. This is your best low-energy option. Bingen itself isn't all that exciting except for its scenic location and its waterfront. Get off the RE train in Bingen instead of Mainz and walk to the river promenade and boat docks. If it's lunch time you can dine al fresco and enjoy a tremendous view of the river. Board one of several cruise boats for a northbound journey past dozens of castles/castle ruins, vineyards, and some very attractive small towns. It's 1.5 hours to St. Goar or 2.3 hours to Boppard, both good places to get off and to later continue your train ride to Koblenz and eventually Trier. St. Goar is the home of Rheinfels Castle. Leave your bags at the TI office (if its open) and make the uphill walk to take a self-guided tour if you like. Or head to a lunch spot on the tiny "main drag", the Heerstrasse.

Everyone has their own style of travel so for me, the thought of traveling after a long, trans-Atlantic flight is not my idea of fun. I want to find a room, shower and maybe go out and walk around, have a meal, sightsee a bit. Traveling on after a flight means hanging around the airport waiting for a train, sitting in the train for 3 hours or more, finding your hotel and then relaxing? No thanks.
You don't have to unpack your bags your first night. Just have the stuff you need for the 1st night on top. For me, this is a no-brainer. You leave the next morning for Trier, well rested and able to enjoy your day there a lot more.

Ms. Jo's idea about not having to unpack is great. If I have a one-nighter after a transatlantic flight, I pack a complete change of clothing in the biggest outside zipper compartment of my rolling carryon, or in my personal item. Toiletries get packed in one of the smaller zipper compartments, or in the personal item.

When we did GAS tour, we stopped and spent the first night in Bacharach. Easy enough to catch trains to Trier next day. St Goar would have been a good option too. I would not stay in Frankfurt - you might as well move on down the line.

Your RS tour will spend some time visiting Trier ( a great little place! )so why not see a little bit of another area. The Rhine spots mentioned or perhaps Heidelberg>

Why not Frankfurt? It is 11 minute away by train from the airport and has so many historical sites to see, it is a shame to be so close to one of the most important cities in Germany and not see it.