What type of VW is it, T2,T25,T4 or T5. On the earlier campers the vents located high up on the rear corners were to assist in ventilating the engine compartment. If it's one of these I'd check inside the engine compartment to see if anything has "died" in there!!
If it's a T4 or T5 with a front engine then can you elaborate on which vents the smell is coming from, as that may give a clue to the cause.
If it's a T4 or T5 with a front engine then can you elaborate on which vents the smell is coming from, as that may give a clue to the cause.