It all depends on the RPG itself. RPG's can be very different.
In Super Mario RPG, there's a lot about skill since your "timing" effects just about every singel battle. The power of almost all the moves in the game can be boosted in some way, for example: A few moves can be powered up by rotating the directional buttons and others by pressing a button at the right moment. Mario's Super Jump and Ultra Jump can do insane amounts of dammage if you're good at using it. Every singel phsycical attack in the game can also be defended against by pressing a button right before you're hit for either half dammage or no dammage at all. Many instant-death moves can also be guarded against this way, leaving the character with 1 HP left instead of being KOed etc. The fact that different phsycical attacks and instant-death moves are done in different ways, makes some of them harder to guard against than others. For example, it's very easy to guar against a phsycical attack from a Goomba for many reasons, but incredibly hard to guard against thos of Culex since Culex phsycical attack is instant and he won't even make any sign that he's going to use a phsycical attack etc.
In games like Golden Sun, it's all about tactics and absolutely nothing else. "Skill" doesn't really matter in those games.
In games like FF, there's usually not that many moves like those in SMRPG. But in a few FFs, there's moves that can be powered up very much by "timing" correctly and being fast. Zell's Duel is a good example.
And also, the ATB system makes "time" become another skill to master. In battles like the "no status effecting junctioning Omega FFVIII challange", you may get a game over if you're a half second late with choosing a command. This, too, makes your "timing" play a big part in the battle.
And it's, of course, a lot about tactics. Knowledge about your enemy and its weakness and what strategy to use in order to beat it. And also your judgement in different situations. If something in your stragedy goes a bit wrong, then you'll have to find a way to get back on track quickly etc.
And finally... having patience and never give up could be considered a "skill".
Knowledge, tactics and timing. Those are RPG skills.