As we approached, I could see a tank bouncing around in the middle lane of the three-lane freeway. Lisa realized what it was, and in a worried tone, said, "That's a propane tank!"
Yikes! There's a potential air-fuel bomb rolling around the freeway ahead of us! A compact car ahead of us dodged the tank, which tumbled end-over-end before coming to a stop. We avoided it entirely. The RV chugged right along.
At Lisa's suggestion, I called 911 and was transfered to the Oregon State Police, where I reported the propane tank sitting there waiting to become a major incident if a car hit it at speed. We passed the RV, which had apparently finally realized that a rather substantial part of the vehicle -- this was an inboard tank, probably mounted under the vehicle behind the driver, Lisa said -- had come loose and gone bounding down the freeway. We heard no earth-shattering kaboom behind us, and we hope no catastrophic failure happened.