Hello. I will be in Germany soon and will be staying in Koblenz. I would like to do a day trip from Koblenz to Wiesbaden, then from Wiesbaden to Mainz, and finally Mainz back to Koblenz. This will be my first time using the DB train system. I've heard that you can purchase a Rheinland-Pfalz Day Ticket and that it will cover all of these routes, even Wiesbaden which is not in Rheinland-Pflaz. Is that correct that the ticket covers the Regionalbahn and S-Bahn trains between Mainz and Wiesbaden, even though Wiesbaden lies outside Rheinland-Pflaz? Thanks for the help!

Yes, you can do your planned route on the Rheinland-Pfalz Ticket. See map of coverage area:

R-P network

However, you will need more time than you are allotting if you plan to visit both cities in just one day. On weekdays you can use the R-P ticket only from 9 am onward, which makes for an even shorter day. And in case you aren't aware, the many fine towns on both sides of the river can be visited by train as well (probably on a separate day.) Braubach for example is just 10 train minutes south of Koblenz, and there you'll find the best-preserved medieval castle in the area, Marksburg Castle.

This looks oddly familiar. I think we have communicated over on TA, though under different names? ;-)

Fussgaenger has already posted the link to the map. Yes, the ticket includes the routes via Wiesbaden, and (I have checked the original German terms and conditions) it even includes the city's public transport network. Since Wiesbaden central station is quite far from the city centre, it will be wise to use it.