According to the updated story, a man and his wife were about 200 yards behind the RV when it occurred. The witness said the RV speed was the same throughout the accident. That makes it sound like the RV driver was unable to hit the brake, maybe indicating he had a heart attack and was dead before the vehicle ever left the highway.
In the original story, the dead man's son did say it was a blowout and that the RV driver's wife was driving a car behind the RV. It will be interesting to see whether the son's assumption that it was a blowout is correct.