I'd be finding a different dealership if they have a salesman that feeds you this nonsense! He probably drinks his own bathwater.....
I am doing severe winter living in my TT with single pain windows. (Yes, I spelled it wrong intentionally)
In any kind of cooler weather, double pane is a much better choice. It is hard enough to keep an RV warm and if you buy a unit that is better equipped for your needs, you will be much more comfy.
I am going to upgrade mine to a more winter friendly unit. And it will have dual windows.
And if one fogs up, it is a very simple easy fix.
Well worth having them.