Get the dual pane windows. Period. The insulation against the heat and sound so far in our 3400 w/ dual panes have been nothing short of amazing. Better than some houses I have been in - on both issues. 115 degree heat index outside - only needed one A/C (and had to turn that down once) and the glass on the inside was actually cool in that heat (not in direct sunlight). Unbelievable sound insulation for a camper OR a house. Well worth the extra $$$ and the extra weight. I am sure the cold insulation will make running around in your Montana in the middle of the winter with a T-shirt and shorts a VERY believable scenario.
Get the dual panes... seriously.