Hello San,
The A frame type movers (Mr. Shifta, Powerwheel etc.) are a very good solution on hard flat surfaces. If you have a slope, gravel, grass or any other surface where you would have difficulty in holding or moving a caravan by hand, the A frame movers are likely to suffer the same grip and driving force problems that you have.
You must consider that you have impared safety on a slope, and the weight of the caravan can override the gripn of the small driving wheel causing a caravan runaway.
The chassis mounted movers have far superiour traction and control on a much wider and realistic range of surfaces.
The choice is yours