Without a controlled experiment using 2 identical RVs, one with single pane windows and the other with double pane windows, sitting side-by-side under identical conditions, I don't know how anyone could give an accurate financial analysis of the reduction in heating and cooling expenses. Whether or not to spec double pane windows is as much subjective as objective - the additional benefits of reduced/eliminated condensation, noise reduction and generally a more comfortable interior weighed into our decision as well.
Rusty