Some manufacturers offer them as an upgrade option. We tried to get them on or 2014 KZ TT but KZ stopped offering them allegedly due to a small number of requests for them. In 2013, they only had a handful of orders for them so I guess it's not profitable for so few orders.
The other thing about dual/double pane windows is that they reduce noise transmission through the window as well.
What is the difference between a dual and thermal pane window? Low-e coating and/or another pane of glass (triple pane)? Never heard of a difference.
Having worked in the ceiling cavity of several TTs, I would say the greatest heat loss is through the ceiling. Lots of voids, batts compressed all over the place (reduces R-value) and no insulation whatsoever along the sides (around 6" or so). They seem to either have no clue how to properly install insulation or don't care. Probably both.