Certain manufacturers have had a long history of dual pane window failures. DRV comes to mind. I have spoken with DRV owners who have replaced some windows twice for seal failure and resultant fogging.
That said:
Personally my dual pane windows are all 2001 vintage Newmar and not a single one has every failed or fogged up at all.
When I purchase my Newmar 5'ver used the salesperson did not show the dual pane windows nor were they ever disclosed to me in any other way so I did not knowingly pay a premium for them.
First camping trip out with new unit I kept commenting repeatedly to my DW how dead quiet inside the Newmar was. It was only after several days on the road that we opened a window and I notice the thickness and construction of the window glass with the seal between the 2 panes that I realized the unit we bought had dualpane windows.
In my experience with the sound insulating value, the lack of cold weather moisture buildup and the heating and cooling insulating values I will not have a RV without dual pane windows from now on and would pay a premium for them.