Dave, maybe the single pane glass is thicker than the dual pane glass, which would make sense. Darker tinting (at the mfr) may also impact the difference in R value.
Still less propane, less electric (for A/C) if paying for the meter, less sound. Also the more glass surface area in the coach the bigger the difference an R value of 2 is going to make in the overall insulation of the coach. The 3400 seems fairly liberal with their windows...
I'm very happy with all aspects of my dual panes. Next test is to start bouncing them down the road to see if they leak...