I remember seeing almost the exact same thing happen while driving my trusty Hillman Hunter about 35 years ago. It was on the A39 somewhere near South Molton, Devon I think. There's a huge dip in the road, where even cars not towing caravans had to go at full belt down the hill to make sure they got up the otherside.
When we got the the crest of the hill there was a Morris Countryman towing a huge caravan going hell for leather down the hill so I stopped to see if he'd make it up the otherside. He didn't. He got halfway up and then slid back down to the bottom and jack-knifed into the ditch.
Sorry to say, but my Dad and I just couldn't stop laughing for ages after seeing that. :lol: