I would suggest that the product you describe (cruise) does not exist. No one can make money cleaning and turning around a sleep-aboard river boat every three days. However, if you use the Search box (top center, desktop version of this board) you will read about various easy, "independent" options (many from Frankfurt) to stay a few nights on the Rhine, in some lovely towns, and to sleep wherever you want. I believe there have been some posts about sleeping in castles, although that wouldn't be my own first choice.
Like many others, we have used the KD line (which is invariably referred to as a river "cruise", although it is a day-boat line), to see the Middle Rhine, the only portion of the river that is really scenic. That only takes around two hours on the water, and is more efficient taken downstream, or northward. There are many, many posts here on that topic.