When I build a 4' wide trailer the only side insulation I use is a foam backed rug, glued to the plywood side walls.
Any trailer that is wider I go with a conventional "stick built" insulated wall.
Its so easy while building to insulate the roof and floor, that its not worth not doing it.
The problem with the descision on weather to insulate or not is after its done (or not done) you will never think of it again. Lots of people never insulate, and are perfectly happy. If you were comfortable sleeping..that's great, if not it was some other reason. i.e. It was just to hot, I left the window open to much, wrong sleeping bag, he/she sweats.
Next weekend 2/9/07 I'm going camping in Central PA. with my trailer. As I look at my thermometer now it says 6 degrees. In the summer I camp on the beach in Southern Delaware with no electric. These are extremes, and not everyone camps like this. But for the few times you do insulation is nice.
My opinion
Happy Trails
Len