In December a friend and I are in Frankfurt. On one of our days there we want to go to Speyer then Heidelberg then Mannheim and then back to Frankfurt.
Am I right in thinking that we can do this by buying a RMV Gruppentageskarte and a VRN Ticket 24plus?
If so where can we get a "VRN Ticket 24plus" online? Or would we have to get off the train just as we got to the VRN region, buy the ticket and get the next train?
We'd be travelling very early in the morning before the Lander tickets would be valid.

Thanks for any advice.