Cedar and Black Locus(t) wood are naturally resistant to that kind of nastiness, and I'm sure there are some other species out there, too, with varying degrees of resistance. Another option would be to prime your wood with something like KILZ Primer to take care of that, although I'm sure that it would lose effectiveness over the years.
Going to double-up on the rockwool recommendation; yes, it's expensive, but it's one of the best insulators out there by far, and it is resistant to mold/mildew, fire & heat, as well as sounds from outside the bus.
Maybe it's just because I like metal, but using some very thin metal sheets on the inside of the bus would have a nice effect as well as resist all kinds of nastiness as well. From Stainless to brass to aluminum, if you keep it insulated and do not directly layer them on top of steel you can minimize or remove galvanic corrosion nearly completely.
I wouldn't use plastic unless I had to, that stuff just keeps getting nastier and nastier the more we discover about it.
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