Something else I was thinking about doing was use something like this (see link below) to insulate the car. Put it behind all the interior trim and headlining, don't know how well it would work.
http://www.ebay.co.uk/itm/VAN-INSULATION-FOIL-ROLL-CAMPERVANS-VW-T4-T5-VAN-10-METRES-50-CMS-/250942074614?pt=UK_Campervan_Caravan_Accessories&hash=item3a6d502ef6
That isn't a bad shout at all! Even just to stop the rattles and road noise, I can see us being a little cold whilst we are there if I'm honest!