Mamiya 6. I cannot believe no one has mentioned this system yet. The most lightweight and portable MF system with multiple lens options, slightly smaller than the Mamiya 7, which is a great lightweight system too. It also has the sharpest lenses in MF.
Neither the Mamiya 6 or 7 are purely mechanical however, both have electronically controlled shutters. If that is what you are after, I'd go Hassy, which isn't much lighter than your RB though.
Mamiya-6 Folding is quite capable of making good negatives if 6x6 is acceptable.