Dual-pane or insulated windows will be hard to find in travel trailers. Most of the travel trailer market is outside the price range of building true "four-season" capability. The windows are only a small part of the cost of building for four season use, as it involves thicker and better insulated walls, heating basement spaces, putting all plumbing in heated spaces; it is a distinct market segment from mainstream recreational TTs.
However, relative to the other costs of building for four seasons, the dual pane windows are relatively cheap, and most of the heat loss in even the most poorly insulated RVs will be through the windows. So if you can get dual pane as an option, or as a replacement, it would be worth the cost, for less heat loss in winter, less heat load in summer.