If you are going to be winter camping when the temperature is less than 20F and if you don't like living in a dark cave, double pane windows keep in more heat and allow natural light to come inside. Our 2001 TC has double pane windows, as well as large double pane skylights and double pane vent covers. No seal failures yet in any of them.
When the temperatures are lower still, reflectix layers covering the windows to keep more heat in. The reflectix on the table window, skylights and vents are removed when we aren't sleeping.
For temperatures dropping below 0F at night, we made the table window triple pane ala Photomike's post. Can sit comfortably next to the window and watch the night sky. Cuts down on wind blowing through the window track drains and crevices when there is a blizzard outside (although they can be taped shut from the outside). Reflectix added when we go to bed.
While there are lots of holes in a TC, most people will find and plug/seal them over time. Thermal pane windows are part of that consideration, or put another way, will the batteries last the night, how many days before the propane runs out and is it warm enough inside the TC to be comfortable and run full utilities for the length of stay you are planning.
Speaking of holes, when manufacturers fit windows into a camper, they are not a close tolerance fit around the edges. While double panes won't have condensation on the windows, the aluminum window frame will still have condensation / ice, despite having a "thermal break in the window frame. Fix by removing the inside trim and sealing up the gaps between the sides of the window and frame opening with minimally expanding foam.
Having a vapor barrier around the windows reduces the condensation that results in the window rotting out mentioned above. You can use the clear heat shrink window film on the inside to make disposable storm windows for windows you want to see out of. For windows you don't look out a removable tape or Dap Peel 'N Stick caulking can seal the window as well.
Have you looked behind the trim on your vents recently? Check out the various winter mods threads for the usual suspects (AC, cord box, propane box, range vent and others).
Seriously, just get the double pane windows if you are camping 4 seasons.