I forgot that around 12 months ago I had another attempt using that method, I got them apart, but the tube was quite short. The way I got them apart was to drill a hole in one end of the metal mandrel tube so I could fit a shackle. Got some flat bar big enough to dril a hole just the size of the mandrel. Bent it into a "U" shape and drilled hole in each end for shackles
to attach a line. The single shackle was tied to a post on my carport at ground level.The other two shackles were tied to a winch that went to another post and managed to winch them apart without damage. This way the tube is being pushed not pulled so it is just a matter of a big enough winch. I used a chain driven engine winch that lifts 1 ton. I'm not sure how this would go for you but work giving a try.