Train from Frankfurt airport to Koblenz

Yes, that is easy to do. You need to buy 2 tickets, first would be 2 RMV ticket to go from the airport to Mainz Hbf, then a 2-person R-P ticket. I think you can buy both out of the same ticket automat at Frankfurt airport Regionalbahnhof, If not, then get the RMV ticket from an RMV automat, and the R-P ticket from a DB automat. The RMV Ticket is 4.95 EUR per person (going direct to Mainz, not backtracking to Frankfurt). The 2-Person R-P Ticket is 30 EUR. If you leave at 8:30 am and after on the S-Bahn, your connection at Mainz will be after 9 am from Mainz and legal M-F. Weekends and German holidays, you don't have to worry about the timing of course.

Excelent, I was hoping that it would work that way. I did not know I could do it without changing trains, that's a bonus.
Thanks.

Yes.

Buy RMV tickets (about €5 each) to Mainz from ticket machines at the Regionalbahnhof station at FRA airport. You can buy the R-P Ticket day pass at the same time. Note that on a weekday, your R-P ticket can be used to travel onward from Mainz only from 9 am onward.

Be sure to board only regional trains as you make this trip. IC, ICE, and EC trains are not permitted with RMV or R-P tickets.

Here's the area of validity for the R-P ticket:
http://www.bahnstatistik.de/Strecken/StrR-RP-Saar.pdf

(Edit... Oh Sam, you are fast!)

I didn't suggest the direct RE trains from FRA to Koblenz because I'm not finding them now. They used to be available every couple of hours. Are you finding them, Sam? As far as I can tell, a change in Mainz is needed.

See my edit. I don't see any direct regional train at this time.

Aha.

I think the OP will find only combo RMV / DB machines at the Regionalbahnhof rather than separate machines.

Nope, they did away with those combo machines at the Regional station at least a year ago, or perhaps longer. They have DB machines and RMV machines.

Or just use the DB app

You can't buy tickets for regional travel within a VV (like the ticket from FRA Regbhf to Mainz) from the Bahn website. Can you buy them using the DB app?

A straight Bahn ticket for two from FRA to Koblenz is €55,80, which is more than the RMV tickets and the RL-P ticket.

Can you buy them using the DB app?

Yes. Look up "Verbundtickets" (no idea how that is translated in the English version) in the Menu.

Hmm. So the Bahns sells Verkehrsverbünde tickets on the app, but not on the website? Bad Bahn.

There is a Regional Express train that goes direct between Frankfurt Hbf and Koblenz. It now apparently goes through, or at least past, the airport stations without stopping, but it used to stop at the airport. I know because years ago I took that train from Boppard directly to the airport. The RE does stop in Mainz and in Rüsselsheim. You can use the S-Bahn (S9) from the airport to Rüsselsheim to catch the RE. I believe the change in Rüsselsheim is an 8 minute, same platform change. The RMV ticket will cover you to Mainz; the RL-P Tageskarte will cover you from Mainz to Koblenz.

Due to construction work at the Regionalbhf, the S-Bahn to Rüsselsheim (and Mainz) now leaves from the Fernbahnhof (maybe the construction is also the reason the RE between the Frankfurt Hbf and Koblenz to does not stop at the airport). [Edit: it is.]

The is some information on the Bahn website about a new S-Bahn station at Gateway Gardens, which is northeast of Terminal 2. Is this station going to replace the S-Bahn stop at the Regionalbhf or will the S8 and S9 stop at both? How will travelers get from the terminals to the new S-Bahn station. Will regional trains still use the Regionalbhf? Will there be three train stations at the airport?

I'm looking at the Google Maps satellite view of the Gateway Gardens construction, and it looks like the S-bahn line through the GG station will connect on the western end to the line that goes through the Regional Bahnhof, so I suspect there will still be S-Bahn service in the Regionalbhf. The GG station looks like it is primarily to serve the numerous hotels in GG.

I've recently purchased verkersverbund tickets and day tickets in Stuttgart, Munich, Nuremberg and Frankfurt a. M. from the DB app.

No problems at all.

Just a note, AFAIR there are two train stations (one quite small) in Koblenz. We were only there without luggage because we couldn't find a hotel that suited us in Koblenz. You didn't mention your overall plans, but consider that while Koblenz is a nice visit at a beautiful spot, it's not an ideal place from which to take a day-boat on the Rhine.

Thanks everyone. Not quite sure I have digested all of this, but I think my main idea is still good, get to Mainz and then use a R-P Day Pass to get to Koblenz. We are in Koblenz for two nights and then will be joining a Rick Steves GAS tour in Trier. Wanted to spend a little time exploring the Rhine and Mosel river valleys first. We already have our hotel in Koblenz so it was just getting to Koblenz that I needed to figure out. I appreciate all the input. It looks like I need to figure out how to get from the airport to the train station.

The train station is in the airport. There is signage in English all over the airport. You will need the Regional station which is under Terminal 1. Just follow the signs. Your train will leave from Platform 3.

"Wanted to spend a little time exploring the Rhine and Mosel river valleys first."

A good choice.

From Koblenz' main station you are 10 train minutes from Braubach, where the never-destroyed Marksburg Castle is located. English tours are at 1 and 4 pm.

https://www.marksburg.de/en/

While in town, have a traditional German meal, maybe al fresco on Market Square?

(Tip for people who are not locked into reservations elsewhere... Braubach is a sleepy, old-world town that is well located for outings by train on the Rhine and Mosel. Rather than paying for your train outings, you may want to seek out innkeepers that offer the VRM Guest Ticket (day pass for local trains and buses) free to their guests. That group includes the Wieghardt Inn on Braubach's Market Square, which has doubles w/ breakfast buffet for well under €100. The link below is a comprehensive list of innkeepers that issue the Guest Ticket - search by town name in the second column. Note that ALL Boppard innkeepers offer this perk.)

https://www.vrminfo.de/fileadmin/user_upload/02052019_Uebersicht_Gaesteticketvertraege.pdf

Unfortunately, there are no Koblenz innkeepers that participate. But then, a town like Boppard or Braubach can be a better base-town choice anyway - still central to both Rhine and Mosel destinations, but with loads of old-world charm and fine river scenery.