When I started learning Ruby (mainly focused on RPG Maker XP), I spent some hours looking Ruby libraries and getting used with the language before analysing the RPG code used for the games. Now, some years later, I reallised that it was not the best way. As far as I can expect from your description of your own programming skills, If you are used with Object-oriented programming code, such as, C++, and have plenty of hours of programming, Ruby won't be any challenge for you, in fact, is probably easier. Typing in Ruby is much more faster because lots of definitions are inherently included in the code, meaning that, on one side, you don't need to worry about them (the compiler knows too much!), on the other side, since you don't have all the controll, depending on how deep you need to go, you will probably have some troubles.
Taking into account the above, I strongly recommend you learning directly from the RPG code, trying to follow the body of the code (using the Ruby guides only when necessary) and getting used with how the RPG Game is coded. This way will immerse you in the Ruby language (you will see how the structures are coded and all those things unfamiliar to you) and more important, in what you will able to do there. Likewise, you will earn the structure of the RPG code which is one of the weight tasks before developing something in the RPG Game. Another suggestion is that you may try modifying easy things of the code, such as, the Message System, and then go further, with more complex modification, involving more classes... This will help you to better acquire which is the role of each class and how they intercat each other.
Well, I hope this will be helpful for you.
I need to get used to working again haha