We’re just in the process of converting our transit van to a camper, and have used this stuff on the sides and roof:
http://www.kiravans.co.uk/lining/carpet-vinyl/thermo-acoustic-campervan-insulation-self-adhesive-7mm-closed-cell-foam-with-foil-inner-surface
Very noticeable difference in sound deadening and prevents condensation forming on the sheet metal. As my wife feels the cold we are going the whole hog on insulation – so I (note it’s me doing the donkey work!) have used sound deadening underfelt as first layer on bare metal floor, then 25mm deep wooden cross ribs, infilled with 25mm kingsman, then 12mm ply on top. We are (I am) gonna use a combination of 50mm kingspan and recycled plastic loft insulation to fill the voids in the side panels, then tape over some of the silver foil bubble wrap before ply lining goes on. (Told you its gonna be well insulated!).
TBH, I reckon a normal warm blooded homo sapien could probably get away with the self adhesive foam and a layer or two of the silver bubble wrap stuff under the ply, esp if your gonna put a gas or diesel heater in – but the floor is worth doing somethign more robust that won’t compress.
Even without the walls and ceiling finished our van is now quieter inside than our 4 year old Octavia diesel.