You need to determine the purpose of that plastic.
If that original plastic was meant to be a vapour barrier, don't use
Tyvek. Tyvek allows humidity to pass through it, and vapour barrier
plastic doesn't. You don't want humidity getting into your insulation
because it will condense and form water droplets right inside the
insulation. As insulation gets wet (or even fills with frost) it loses
it's ability to insulate.
If you don't know what the plastic is for, and the trailer manufacturer
doesn't give you a credible answer, then I'd play it safe by using an
insulation that doesn't need a vapour barrier because it's impermeable
to air, such as expanding foam or extruded polystyrene insulation. That
way, you side-step the need for any plastic on either side of it.