I love rangefinders and I love film. I travel a lot, not on motorcycle, but in mountainous regions, and I work parts of the year on a boat. I know South America a bit, after a total of 14 months road tripping there.
If I was to go back for a long trip, I would not take a rangefinder. I have used M6, M7, M8, Zeiss Ikon, and now a Bessa III. All broke down on me in crucial moments, mostly mechanical issues. I still use them, but for documentary work in non-remote places only. All mechnical is a nice idea. Reality is, however, that they break too. Sure, an M2 can in theory be serviced a lifetime. Who will do so in town xy in South America? My M7 failed (mechanical, mind you) in Georgia, Leicas CS was useless.
Further, vibration would concern me too much. Imagine your RF de-aligning, and you notice that not until you get your films back. Ouch. You will likely knock the camera against something during the trip at some point...not knowing how that affected the RF mechanism would make me nervous. A M9 mitigates that a bit, as you can check results, but digital Ms introduce another layer of fragility and problems (loved my M8.2, the dust on the sensor not so much).
Robust for such a trip in my books is Contax. It it was all film... A G2 with the 45mm + 21mm and a Contax T3. Two bodies is a must, one non-RF and ideally P&S.
Personally, my choice would be Fuji something or Leica X-something (those things are tough, really) and a Contax T-series camera. Mixing film and digital is perhaps not desirable initially, but I have come to appreciate the variety and this way you have a back-up without redundancy.
(PS - on my SA trips I used Pentax Espio, Sony DSC V1 an Contax TVS III. All without problems)
Good luck!
Peter