Darrow:
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.