Hello Luke,
Most of what needs to be said has already been done so, but perhaps a little wont hurt.
You obviously appreciate that twin axle caravans are more difficult to turn, and without the muscle of a tow vehicle it can be very difficult to do it by hand.
I did some work for a major manufacture, where caravans were tested in a special facility. It was always tight to get a caravan loaded, and on number of occasions I had to manoeuvre caravans within their own length to get them in and out by myself. The floor was a painted concrete and whist single axles were comparatively easy, twins were literally a real pain.
Despite jacking the jockey to maximum height to relive the down force on the front pair of wheels, and the optimum surface, it was a major effort to manoeuvre the caravan in to position. We found that adding a little water to the surface under the wheels helped, but it equally it was a major slip hazard if you had to walk near the area. These days it would not be allowed under Health & Safety.
With such difficulty on a plain surface, it was even more difficult on less good surfaces such as the tarmac yard, unless you had plenty of room and could take long distances to achieve the turn.
Now I do not know what the ground is like where you need to manoeuvre your van, but bearing in mind the surface conditions can change with rain or falling leaves etc, working single handed or even in pairs, moving the caravan within its own length will still be very hard.
If your surface is hard and flat, then you may find that a mover that fits to the 'A' frame will be more than sufficient, but if the ground is uneven, slopping or tens to get slippery or loose like gravel, then the A frame devices will not be suitable. For these adverse conditions you need the axle-mounted movers that can maintain grip and control under these conditions.
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