of course as a counterpoint...
There are companies out there that have been manufacturing spray-in foam insulation for Trucks, RVs, commercial trailers etc, for over 30 years. It seems they have figured something out. I suggest you give one of them a call and explain how you want to use the product and get their opinion.
I am looking into
this company But I am not quite to that point in my restoration. Probably next summer/fall.
One question I still need a good answer for is this. If one fully insulates with a spray-in foam insulation and thus removes much of the "flex" in the trailer will there be unintended consequences like excessive bounce or sway? Will it force more suspension activity than it is designed for? I don't know the answer to this.