I would be careful where you place the plastic sheeting. Typically the vapor barrier (plastic) should go on the warm side (in winter) of the insulation. In a wood framed home, it would go over the studs, under the drywall. This is to prevent moisture build up in the wall from condensation. If you get it in the wrong place or have multiple vapor barriers you can get mold or rot within the walls. Having the vapor barrier to the inside of the wall will help prevent the moisture created from the occupants breathing, cooking and heating inside the coach from penetrating into the wall and insulation (rigid insulation absorbs much less moisture than batts, but is still susceptible) causing damage. It is surprising how much moisture can be created each hour by a couple of people breathing, also all LP powered appliances (stoves, heaters, etc) will create moisture as a product of combustion. Using a proper vapor barrier (with joints and edges sealed) and removing the moisture from the interior (venting/fans/dehumidifiers etc) will go a long way to providing a healthy environment for the interior of the coach.