Which insulation to use

[possummamma]

I am looking for any and all advice on what product I should use for insulating the roof of my 92 Land yacht MH.

This is a total roof replacement. I will need to sandwich the insulation between two pieces of plywood or PVC board. Seems to me it needs to be somewhat pliable to follow the arc of the beams.

The beams are hollow 1.5 square. I have to fill in this much space so the fill surface is flush with the cross beams in prep for the new rubber roof.

If any of this makes sense, please post your suggestions. I did search the forums, but the latest post I found (not to mean I didn't miss any) were dated 2006. Just looking for the latest and greatest from all you brilliant people out there.