The two most effective changes (improvements) over those years was the addition of mylar bubble wrap in the underbelly and 12 volt heat pads on the holding tanks. Some Cougar models have the option of dual pane windows, but the way you asked your question: "... has changed between the 2007 models and 2014" are you looking at buying a model in those years or ordering a new model, built to your specifications????
If you're looking at a used trailer, changing single pane windows to dual pane is significantly more expensive than you'd imagine. So, it "sort of becomes" what you find is what you get'.....
In short, there were some improvements, but not enough to consider a 2014 trailer "energy efficient". Essentially, you'll use enough propane at 14F trying to heat a 30' travel trailer as you would to heat a 1200 sqft house. If the trailer is going to be on a "semi-permanent site", you can do some modifications like skirting, sealing the slide seals better and even using rigid foam sheeting under the slide and on the slide end walls to help insulate better.
2015 F250 6.7l 4x4
2014 Cougar X Lite 27RKS