Please share experience if you traveled as a single/solo.
See also this current thread (in the General Europe forum): https://community.ricksteves.com/travel-forum/general-europe/solo-traveler-which-tour.
The Thread that Laura provided should answer most of your questions. I always travel solo and the tours have been great!
I love the RS tours as a solo. If it's of interest to you, I did a Trip Report on the 21 day Best of Europe I enjoyed as a solo traveler.