I too had a problem with a "fogged" thermal pane window on the door of my 2007 Damon Tuscany. I notified Damon and they shipped me a new door window (free of charge ) which I installed myself. Not a big job if you are just a little bit handy. It will take 2 people however.
After some discussions with others, it appears a seal on the double pane fails and moisture gets between the 2 panes causing the fog. This can be caused by people adding window tints to the windows or when storing, putting the reflective material in the windows to keep the heat out. The traps the heat between the window and literally melts the seal. While neither of these were the cause of my problem, seals can fail for any reason.
If you are thinking of adding a tint film to your windows, read the box first. Most will tell you not to apply to thermal or dual pane windows.