Stephen - this is the sort of material you should be using (well i decided to use anyway)
http://www.ebay.com.au/itm/Sound-Dea...item27e4240e20
when doing my back floor i moved slightly from the conventional way of sticking the insulation to the floor
i stuck blocks of 2x1 to the floor with liquid nails - painted the floor with sound deadening paint -measured and cut sheets of marine ply to form a false floor - i then stuck the insulation material to the underside of the timber floor
so in effect i have rustproofed the van floor - provided an air gap to reduce effects of condensation - provided sound and temperature insulation - can screw the floor to the wooden blocks without going through metal and creating rust points
insulation can be stuck directly to wheel wells and inner cavity walls (its self adhesive)
Note - all joints are covered with an aluminium tape to prevent heat/sound leakage
geoff