I think your plan sounds lovely. My wife and I spent a week between the Rheingau and the Mosel and never lacked for things to do. I suggest that you get a GPS, as that makes it easier to take the more scenic, direct roads instead of being on highways the whole time, with the bonus that you can't get lost.
I have some tips to add to the others you've received. First, I really enjoyed the self-guided tour of Kloster Eberbach, outside the Rheingau village of Hattenheim. An old abbey (cloister), they claim to have invented Kabinett (then called cabinet). The wines today are not spectacular, but some are good. We also had a lovely lunch at Schloss Vollrads, on the patio, and did not need reservations. Most of the wineries had posted hours, but if you are interested in tasting after hours, you might try a "vinothek" which is basically a wine bar.
If you make one advance lodging reservation, go for Gaesthaus Pruem in the Mosel town of Wehlen. Very large rooms, tastefully modern, an excellent breakfast, and the "mini bar" is a self-service wine fridge. Very, very reasonably priced, but they book up. http://www.sapruem.com/page.php?p=30000
I have yet to visit Alsace, but I can recommend a visit to Christine Ferber's store in the Alsatian town of Niedermorschwihr. Never has a town in France sounded so German! Truly magical confitures, admittedly at dear prices. http://www.christineferber.com/Christine-Ferber.html