Normally, I would have recommended you not take a guided tour, but do it yourself, but you seemed intent on having a guided tour, and the Romantic Road Coach is the only way I know of to do that. I believe Rick does recommend the Coach.
If you want to do it yourself, you should familiarize yourself with the German Rail website to get train and bus schedules (the Bahn site gives the prices of train tickets, but not bus fares, which will be nominal).
Start by taking regional trains to Rothenburg using a Bayern-Ticket. Spend a night or two in Rothenburg, then take buses from the Rothenburg Bhf to Dinkelsbühl (there is no train station in Dinkelsbühl; you'll have to get there by bus). I'm not sure if you can use a Bayern-Ticket for all the buses that day, but in 2007, I spent 8,60€ for the bus between Dinkelsbühl and Rothenburg. Stop in Dinkelsbühl for a few hours, then take the next bus to Nördlingen. In 2007, I paid 4,50€ for the bus between Nördlingen and Dinkelsbühl. Spend at least one night in Nördlingen. Nördlingen is Rothenburg's less touristy little sister. It has a lot of Fachwerk buildings, an almost complete wall with a Wehrgang you can walk on, and a church tower you can climb for a view of the town and surrounding land. However, it lacks a Christmas shop and a Crime and Punishment Museum.
You might want to take a side trip to the Harburg castle, 15 minutes by train to the south of Nördlingen. It's about a km walk to town from the Bahnhof, then a short steep climb up to the castle, so you will want to do the trip without luggage (there are no lockers in the Harburg Bahnhof).
In my opinion, the bus route from Rothenburg to Nördlingen is possibly the best part of the Romantic Road because it features three walled towns and the buses follow route 25, the official Romantic Road. After Nördlingen, I would just take the train to Füssen, with a possible stop at another Romantic Road town, Augsburg.
BTW, I would not put very much credence in the reviews on TripAdvisor, which has been widely criticized for fake, planted review posted (good one) by companies and (bad ones) by competitors. A branch of the Italian government recently tried to fine TripAdvisor half a million euro for allowing misleading reviews (Ref). TripAdvisor only avoided the fines by claiming that they had adequately warned readers that their reviews weren't necessarily accurate. In particular, I spotted an erroneous bad review of a place I stayed at in 2013. The reviewer described the location of the place in the reviews, and her location was far (over 100 km) from the actual location of the property. It might have been an honest mistake; she stayed at another place with the same name, but the place she reviewed was not the one on TripAdvisor. I've tried numerous times to report the review, but T/A doesn't respond.