A couple of things come to mind after reading the above posts:
Dealers want to sell what they have on their lot. If they have a lot of price point units they are going to say anything to move them. Price point units won't have dual pane windows or a lot of other nice options.
I full time and have dual pane windows. Don't think they are just about keeping the cold out during winter. We had a couple of nights back in January on my home base in Alabama where it was 9F. I was nice and toasty inside and zero condensation on the inside of the windows. I spend summers on the Florida gulf coast and dual pane windows allow me to remain cool when it is 95F outside and 100% humidity.
I would not even consider an RV with single pane windows.