I looked at 19.44 and it clearly shows both slip plates touching each other. I can also see the thicker portion of the kingpin just below the slip plates. I see just below that the narrower portion of the kingpin and then below that the slide bar.
It is hitched for sure at 19:44! If it were "high hitched" you would see at least a 1" gap between the two slip plates.
When he hitched the kingpin was NOT fully forward in the throat of the hitch so he had to push hard to get the handle to slide past the kingpin. At this point there was enough rearward tension to keep the bar in place and the handle in to look like all was well.
His wife started filming him pulling away then had him stop. During this initial time of pulling ahead if high hitched that sucker would have dropped instantly! The video starts again and almost immediately the RV drops like a rock. Reason is when he stopped the tenion of the kingpin against the slide bar released and the slide bar released and once pulling again it drops.
CASE CLOSED!!!
After reading some other ideas I will agree it could be that the bottom of the slide bar was being forced past the top edge of the lowest thick flange of the kingpin. Either way results in the same scenario that caused the drop.
CASE CLOSED!!! Again