When it comes to small motorhomes dimensions marginally different means a lot. The Roadtrek RS Adventurous is 22'-9" long not 24' long or 24'-6" long for the Serenity. The width is 6'-8" vs. 8'-1" for the Serenity. You would be a lot more nervous parallel parking a Serenity on the street and sticking out and would have a hard time fitting inside the standard 24' long space. The Serenity breaks the 10' height barrier and believe it or not you then have to be cognizant of garage door heights. You could get in my local self car wash with the Roadtrek but not the Serenity. There are simply a lot of places you can drive that Roadtrek where a Serenity cannot go. Parking is more critical than you imagine. The Roadtrek is a true van thus a true Class B. The Serenity is built off a cutaway chassis thus is technically a Class C. They share the fact they are both Sprinters and have the same wheelbase.
If you like the Serenity then you owe it to yourself to look at all the small Cs and make an apples to apples comparison. You may not be a B person at all if the Serenity has more appeal over the Roadtrek. You can't do this with just web research. You have to get out and look at them, test drive them, and take a lot of quiet time sitting in them and imagine how you would use them. Don't just look. Go through some pretend motions. Rentals are pretty hard to come by but it would be nice if you could only a lot of rentals are nothing like you would consider.