Is this a feasible itinerary?

so you are flying home from both Munich and Montpellier?

I'm confused.

https://community.ricksteves.com/travel-forum/france/feasible-itinerary is what has confused me.

Hi,
I don't understand why you're spending so much time in Frankfurt, and any time at all in Mannheim.
Upon landing in Frankfurt, you can head straight to a town on the Rhine such as Mainz, St Goar... and allow three nights for exploring the area. No need to go as far north as Koblenz. You can leave the area on day 4, and I would stay in Heidelberg rather than Mannheim. One night is enough, maybe 2.
You can then rent a car in neighboring Mannheim, and allocate one extra night in the Black Forest, and one overnight in Salzburg.
I'm also not sure Meersburg deserves the overnight. I prefer Lindau which also makes a good base for Neuschwanstein / Hohenschwangau since you'll have a car.

In Koblenz I suggest too...see "Deutsches Eck" from an aerial view in the Festung Ehrenbreitstein.

Take the S-Bahn from Koblenz Hbf to Ehrenbreitstein. ...lovely view for photography.

You've listed two Day 10s.

I'd think you'd want to dump the rental car the day you get to Munich, not one day later.

Feasible? Yes.

Some thoughts:

4 Koblenz Eltz Castle: Unclear... does this mean you are choosing to STAY in Koblenz or VISIT on your way to Eltz? Eltz is not in Koblenz.
5 St.Goar Rheinfels Castle: Same question about St. Goar. And is Rheinfels the ONLY thing on your agenda this day?

• Eltz and Rheinfels are close enough that a single town would be just fine for overnight.
• It's surprising that one of the most popular activities - a Rhine cruise - is missing from your agenda. No interest in that?
• Since you're obviously into castles... the most authentic medieval castle experience in the area is MARKSBURG Castle in Braubach (tours of this never-destroyed, intact castle, in English, at 1 and 4 pm.) I'd put that at the top of my list.
• Koblenz is mostly a "meh" city - very modern, just a small old-town zone... What's unique about the Rhine Valley is its collection of small old-world towns and the atmosphere they provide. St. Goar is small and VERY scenic and makes for a convenient base town for visiting Eltz, Marksburg and other places over 2-3 nights; waterfront rooms offer great views:

https://de.wikipedia.org/wiki/Liste_der_Kulturdenkm%C3%A4ler_in_Sankt_Goar#/media/Datei:Mittelrhein_-_Flickr_-_tm-md.jpg

https://t-ec.bstatic.com/images/hotel/max1024x768/900/9002574.jpg

Boppard (just to the north of St. Goar) is also very old-world-attractive and an excellent base town - waterfront hotels there as well. Its chairlift ride offers great views of the valley.

St. Goar and Boppard both have FERRY CROSSINGS that run all day if you want to cross the river (to visit Marksburg/Braubach or elsewhere.)

Other places you might want to visit:
Oberwesel
Cochem
Bacharach (Not covered on the VRM guest ticket however, see below.)

Unless you have mobility issues, I would use the trains in this area instead of the car. If you stay in Boppard, your train outings to St. Goar, Eltz, Marksburg, etc. are free of charge with the VRM Guest Ticket that comes with every Boppard hotel room. Trains cover this area VERY thoroughly, scenically, and efficiently. See Rhine Rail Map with train lines on both river banks; see also Mosel Rail map for journey to Eltz.

6 Manheim Manheim Castle
7 Manheim Schloss und Schlossgarten Schwetzingen OR Schlos Heidelberg castle

I agree with the comment that Mannheim isn't all that super - I suggest you transfer a night to Boppard or St. Goar and get by on one night in Mannheim.

cut the extra day in Frankfurt and Manheim. Spend more time in Bavaria.