Some Mamiya RZ Pro II / Pro IID bodies have issues with batteries. In fact, Mamiya allows a small drainage of battery when the camera is off (and it's ok with their specification.. it doesn't make your camera eligible for warrnty repair). I was shocked by how fast the battery is depleted in my RZ Pro IID and I serviced my body. Then I was told that my Mamiya body drains battery even when it is in 'off' mode. It would eat up a high quality battery in a week or even few days. It is a design glitch by Mamiya, because they don't consider it a flaw. The solution to this problem is to keep your camera in 'mechanical mode'. The switch in Mamiya allows 3 positions: On, Off and full-mechanical operation. In this third mode the battery is cut off- as you probably know, this mode is for the situation in which you want to shoot a photo but the battery is depleted (you can't change the shutter speed, it's fixed at 1/1400 but you can take a photo in emergency situation).
Since the day I started storing my camera in 'mechanical mode', I can run a single battery for a looong time.