I'm sure the two reps mentioned only had single pane models on their lot.  RV salesmen like car salesmen make more money on what they have in stock.  Nothing against any salesman, but it is just the way their system works.
I have dual pane windows and I full time.  Going on 4 years now.  I just came through the coldest winter so far with temps down to 9 degrees and not a single bit of condensation inside my windows.  I spent my summers on the Florida Gulf Coast where it is hot and never a problem staying cool with my two a/c's.
Dual pane windows are worth ever dime and I would not buy an RV without them.