Cochem - Rheinböllen - Sprendlingen - Wörrstadt - Gernsheim (for the ferry) - Heidelberg - then a choice of taking the Nekar river road to Gundelsheim or the SE route via Wiesenbach / Helmstadt to Gundelsheim - then on to Rottenburg.
I don't know if you've got accommodation sorted, I've stayed in this place a few times over the years, it's near the town square and they have private parking around the back. It's been a few years since I was last there, hopefully it's still as good.