I don't know if you got your answer since then but well, I can answer because I followed the same pattern for my own engine.
RolePlayingGameData is a project that contains all the data classes of the game. These classes are referred in RolePlayingGame, the main project, which contains the engine / the game. For some purposes (but mainly the read/write of XNB/XML files) all the Writers (and Readers could have been placed here too) have been put in a separate project (RolePlayingGameProcessors).
For my own project, I separate the things differently because I'm realizing a kind of generic engine/framework to make real games later more easily. But I still have three projects : a first one that contains all the classes of the engine, a second one that contains the specific classes for my test game (and other games later would be another projects) and the last one that contains all the ContentWriters and ContentReaders used in the engine / the game.
Then, it's up to you but if you want to add functionnality such as multiplayer game, you may have to change things in RolePlayingGame (maybe by adding new screens to manage the inputs, the multiplayer rules and so on) and in RolePlayingGameData to add core data classes (a class to symbolize your multiplayer character for example).
Hopes it helped !