I bought a wrecked 20 foot full sized travel trailer. I dismantled it for the parts, it had fiberglass insulation, 1x2 and 2x2 lumber. The trailer was 20 years old too, the fiberglass was still doing pretty good. There was no structural value to it. Fiberglass is being used all the time commercially.
I used that reflective bubble insulation, it has many brand names, it come in rolls at the hardware store, but its bacically bubble wrap with aluminum on both sides. really light weight, just cut it with scissors, no fibers floating in the air, no VOC's it couldn't be easier. I put in two layers. ....Bob