When I was looking at an Airstream, they also did a lot of window treatment, making fairly small rectangles. It makes sense why they did that -- the single pane rear windows do radiate a lot of heat.
As for the crack between the two doors, I've wondered about some piece of thick curtain fabric on one side, Reflectix on the other, and Velcro to hold it in place. For the bottom, one could do similar, but even a beanbag filled tube would work to keep drafts at bay.
As for insulation, my favorite is Reflectix or just the humble closed cell Styrofoam sheets, perhaps with a foil backing. It is decently mold and water resistant, and seems to work well enough. Aerogel insulation comes in sheets, but I've not read anyone using it, so I'm guessing it may be too expensive for general use. I dislike fiberglass insulation immensely just because every property of it is icky... be it itching if it gets on your skin, the fact that rodents love nesting in it, and the fact that water will compromise it. I am also not a fan of open cell spray insulation for exactly the reasons mentioned above.