I use xps material whit fiberglas as a sanwich compound for the walls, and inside I add a 60 mm xps tablea , at all, walls and roof, because It is one space betwen the metal chasi and the internal walls, after that is 3 mm fenolic compac platea as a finish material, this is the one is used on the gym's to make toiletes and walls It is waterprof and much more resistent than plywood, total walls are 90 mm , the two problems I see for that in a van are the space , because You use a lot of room inside and the height, xps is very light, but the compact wood no, keep in mind, just the 3mm compact for all the walls and roof are close to 200 kg ( Also internall walls to divide the WC and bed) , but I Think is the necesary amount You need on the walls for this material, xps here is very very cheap , the k is very low as a 0,034 in 60mm
