The definition of RPG is incredibly arbitrary since it has nothing to do with role-playing so everyone's going to have their own. Generally speaking I'd say there are some games everyone who isn't trying to be a troll agrees are RPGs (Dragon Quest, Final Fantasy, Ultima) and games that everyone agrees are not (Super Mario Bros., Madden, Doom), and I guess from there what is or isn't an RPG depends on how close it falls to a given genre pole? That's what I do, roughly, anyway.
Personally, I don't consider Dark Souls an RPG as it's waaay closer to DMC/Bayonetta (emphatically not RPGs to me) than anything I'd consider definitely an RPG; Castlevania and Zelda are in similar boats. On the other hand, Mana games do qualify as RPGs to me, due to things like having multiple party members and menu use for combat (spells)... even the storytelling style/progression. They're certainly more like Final Fantasy than they are like Zelda to me, let alone things which are even "less RPGish". But if someone else's definition is a bit different and encompasses different things then that's certainly not worth name-calling over.