Is the Rheinland-Pfalz Ticket good in Hesse?

This is not an exact answer to your question, but only a small portion of the right bank of the Mittelrhein sits within the borders of Hessen. The border lies just beyond the town of Lorch. Everything else downstream is Rheinland-Pfalz until you hit Nordrhein-Westfalen just south of Bonn.

It's also somewhat confusing, but portions of the region (mostly upstream of the Mittelrhein) are still colloquially referred to as "Rhein-Hessen", reflecting the pre-war borders, even though they now lie within Rheinland-Pfalz. Along the same line, the region further south around Mannheim, Heidelberg, Speyer and Ludwigshafen is often still called the "Kurpfaltz", even though that particular political entity disappeared during the Napoleonic period.

The R-P ticket is good in Rheinland-Pfalz for the most part, but there are certain overlaps into other states, and that part of Hessen you mention that lies between Braubach and Rüdesheim (on the right side of the Middle Rhine) is indeed covered.

It's easy to check the R-P coverage with THIS MAP.

Thank you for the information - the linked map is very clear.

I did not see the map on the English language section of the DB website, but I found it on the German section and learned a new word - Streckenkarte - "stretching" map?

I'm guessing a more accurate translation is - "coverage" map?

Good news. It saves the 4 of us an interesting amount of money since we would like to go down the right bank so that we can see the Blücher museum as well as get a different view.

STRECKENKARTE:

"Strecke" just means "line" as in "rail line" in this word and not "stretching." But I imagine our English-language use of the word "stretch" as in "stretch of track" or "stretch of road" was passed down to us from the German "Strecke."

The verb, strecken, does mean to stretch, but the noun, die Strecke (sing) means distance, route, or (rail) line.

Note that, like many Länder tickets, the Rheinland-Pfalz-Ticket is valid for a number of lines outside of the Land. The RL-P ticket includes the Saarland. It can be used for travel to the Bonn Hbf in Nordrhein-Westfalen and to Mannheim Hbf and to Karlsruhe Hbf in Baden-Württemberg and for the stretch of track on the right bank of the Rhein between Mannheim and Karlsruhe.

Conversely, the Baden-Württemberg-Ticket can be used from Karsruhe and Mannheim to Wörth and Ludwigshafen respectively, in Rheinland-Pfalz, and for the stretch of track on the left bank between Wörth and Ludwigshafen.