Thank you John for resurrecting this classic film clip.
four points though, firstly and importantly, 85% is not a rule it is only a guideline. Continued references to it being a rule is unhelpful.
Secondly I suspect it was not filmed in the UK,- (Car driving on left of road, driver on right, and caravan door on the right), so the 85% guideline would not apply anyway.
And lastly, The caravan probably exceeded the car manufacturers permitted towing limit.
Notice the canned laughter on the clip - was real or a stunt?