Play style is wrong. No need aa on rm bb5. rm bb5 is op as it is. Play like usual and you will be fine. Unless your play style on IJN,KM and MN BB5 are also wrong. 

Then you need to revise.  

Post nerf, the RM BB5 is arguably the worst BB5 in the game. It was OP before it was nerfed, but is now somewhat UP. But not needing AA is right. Either use HH or put reps in the T slots.

As to OP, the thing to realize is you have no real advantages to press. It used to play like a the BB4 on steroids, but now plays more like an H39/44 without the range advantage. You are pretty much average to below average in almost every area, and what advantages you do have are so small they might as well be nonexistant. Because of this, the ship should not be used aggressively. You have rather poor spread, the second lowest salvo damage, and the reload and hang time advantages are miniscule. The low angles also make your guns very easy to bounce. You are also not particularly fast or agile, and your range is relatively poor.
So, how do you fight with all that? Never, ever, EVER get into a slugging match or rush someone who can see you and isn't distracted. Instead, play defensively and let them come to you. Against BB4+, only rush in when part of a group or the enemy is blind, and only if you know your angles. Against BB3-, use your range and be wary of rushers. The ship is not easy to play like it once was, but it's still useable if you play smart.