I couldn't imagine being gone on a trip that long and seeing what I can only imagine you will be seeing and not having a lens wider than 25mm. I'd definitely be taking my 18mm f3.8 Super Elmar. Maybe in your case a 15mm f4.5 III Heliar might be the right answer for a few great opportunities?
I am no multi-frame "stitcher" of digital files. If I need a super-wide shot, I'd rather get it with a super-wide lens.