[JRTJH]

Opening roof vents on today's RV's while towing with today's tow vehicles is "fairly clean" however, towing with a naturally aspirated diesel "of yesteryear" will cause the DW some significant grief if you try to tow with the roof vents open. You will create a vacuum inside the RV and if you're unfortunate enough to be towing at elevation, the black smoke created by the diesel will be drawn into the RV, coating everything inside with a "greasy, black film" that smells remarkably like diesel exhaust.

Don't ask me how I know this, but one trip through New Mexico with our 93 F250 was enough to motivate me to order a new truck with a turbo...