my experience is that soundproofing is a function of weight and density.
i've had good luck with 12x12 dynamat-like products, sourced from auto body
supply shop. heat with heat gun the make 'em malleable, peel and stick on
any reasonably clean surface. inside the van should work just as good as
outside for soundproofing. just my 2 c worth, and probly worth no more than
that. david