Equipment for trip to Rocky Mts
What else to rent depends on the types of landscapes you like to take. The 12-60 is quite nice for something like this (at 55mm):
But I used all 200mm of my 50-200mm to shoot this:
And 300mm for this:
A cityscape at 11mm:
You might want to consider the 1.4TC, but be prepared to shoot only in very good light with your 70-300 and definitely on tripod. I'm not sure how sharp the images will be, but on the other hand, this might allow you to at least take shots of wildlife at a distance. I just bought an OM 500mm f.8 (don't have it in my hands yet) & I'm going to try to see how it works with my 1.4 & 2.0 TC & E3. I would not suggest the 2.0TC for you--it definitely reduces image quality--but I do use it frequently with my 50-200. That said, I don't think the long end of the 70-300 will work well with the 2.0TC.
Regardless, have fun on your trip & be prepared for snow!