I took the van into town today for a run (50km round trip) and had the tailgate up all the way. With windows down my wife was whinging about the fumes, wound up and there was no problem. With the fan on it was even better. With the tailgate shut I have no fumes even with both windows wound down, so I guess my 30 year old tailgate seals are still good.

It does seem to be an individual issue for different vans and/or exhaust exit points.