The full regional fare for Füssen to Oberstaufen is 26,70€/P.
For Füssen to Oberstaufen, there is a Regio-Ticket Allgäu-Schwaben at 22€ for one, 28€ for two. The Bayern-ticket is 3€ more (for 1 or 2), but would also cover any bus transport in Füssen to get to the Bhf; the Allgäu-Schwaben ticket covers only the train.
For FRA to Rothenburg ob der Tauber using an RMV ticket and a Bayern-Ticket, you need to get from the close-in Regionalbhf at the airport to Frankfurt Hbf, either by regional train or by S-Bahn, then take one of the hourly Regional Expresses (RE) from Frankfurt Hbf to Würzburg. The RMV ticket will cover you from FRA to Frankfurt Hbf plus on the RE to Kahl am Main, the first station in Bavaria, at which time you can start using the Bayern-Ticket (no need to get off the train, just change tickets).