Floor insulation??? Now there's a thought. Maybe 1" of that hard blue foam. One 4X8 sheet should be enough to do the job. Slice it in to manageable pieces so it could just be put together like a puzzle. You could n make it interlocking so it would act lie one piece. Put the throw rug on top.
At the gym we go to has this soft rubber interlocking flooring that is about 1" thick. Looks good and can be easy remove and cleaned. Maybe a possibility?
We do a lot of cold weather camping. We use the foam interlocking tiles on the floor. We used 3 foam tiles I purchased at Garden Ridge in Ft Worth,TX, for less than $20. I cut them to fit over our vinyl flooring int he winter. They seem to make a big difference in keeping the Casita warmer and It is nice not to put our feet on a cold floor. I also put a couple of small rugs on top of the foam.
We use the Reflectix on our windows. The stuff is cheap and readily available. I have ours cut so it fits snugly over the window and within the metal window frame. They have traveled will & not fallen out when we are in transit. We felt a immediate difference in cold air "pouring" off of the windows. The Reflectix is also very helpful in the extreme Texas summer heat. My "unscientific" opinion would align with what Bobinyelm indicated, having no gap between the window is important. We replaced our metal Casita blinds with some vinyl blinds from Walmart. In all we came in under $100 to improve the temperature in both extreme heat & cold.