So buses, popup trailers, travel trailers!cargo containers and other such artifacts are all pretty poorly insulated.
Insulating them on the inside eats up living space and forces you to live with any off gassing.
What if you wrapped the outside in 6 mil + plastic, glued stone wool insulation to that, and then glued on a final layer of aluminized bubble wrap?
Windows could be cut out, maybe they would be 10-20 layers of bubble wrap between two layers of clear plastic.
Aside from the windows, no great amount of flammable material would be added.

Just an idea, let me have some feedback!

I would be very concerned about the wrapping breaking down as it is exposed to sun and wind. I think you'd need to add some kind of durable covering over the insulation, either wood or metal. I've not had good luck with any plastic products exposed to sun.

We have not tried too much in the way of insulation, but by enclosing our 5th wheel trailer in a "high tunnel" we have stopped convective heat loss by more than 30%. We have been living in our trailer for the last 8 years in our harsh Canadian winters with up to 4 feet of snow and down to -40F. It is literally a green house, on sunny days it will get above freezing, that greatly reduces heating requirements.
The last two mild winters we got away with only one 500 Gallon propane fill. The -40+ years takes two fills. The hot water heater and furnace are vented to the outside of the tent. We have a single 15 amp circuit for our electrical requirements, hoping to switch to solar in 2016. Plastic used is a single 20' x 100' roll 6 mil uv stabilized builders plastic. We put it up before the first snow and take it off in the spring.
A new roll every year, the used plastic sheeting goes to green house and other uses. Cost was $80 CDN is now up to $130 CDN. Hop vines grow up the structure in the summer giving us shade.