Hello Andrew,
Trevor has suggested an 'A Frame' mover. To help differentiate between the two types the jockey wheel replacements I call them 'A Frame' movers and 'Main Axle' movers hopefully for obvious reasons.
'A Frame' movers do have their place and for some people they are just the ticket, but they do have some important differences and possibly more importantly some particular safety considerations.
Most of these movers have plenty of power to move a caravan, but its how the power is coupled to the ground. The ability of a mover to grip the surface it sits on is determined by how much force (or weight) pushes it to the floor. With a main axle mover you have roughly half the caravans weight on each wheel, where as an A frame mover only has the weight of the mover plus the caravans nose weight.
So for a rough example an 'A frame' mover weighing say 30Kg (mechanics and battery)is connected to a caravan of MTPLM 1000Kg, with a nose weight of say 70Kg. The combined load on the mover is 100Kg The ratio of mover load divided by MTPLM is 100/1000 or one tenth. So on a dry surface the mover would just about hold the caravan on 1 in 10 gradient. But because the weight distribution of the van is affected by the gradient, if the van was pointing up the hill, the load on mover would be reduced so it would not hold, but point the van down hill and the additional weight transference due to the slope onto the nose would increase its holding capacity.
Then factor in poor surface conditions such as water, leaves mud or ice, and the ability of the A frame movers is significantly reduced.
The above applies to where the caravan is not moving, but factor in the kinetic energy of a caravan in motion and even at a 0.5m/s the ability of an A frame mover to stop the caravan is further compromised.
As a rough guide, if you could not stop a caravan on the slope by yourself, then an A frame mover has about the same stopping capacity as an adult attempting to do so.
So I suggest that A frame movers should only be used on virtually flat hard surface.
Other considerations are that you cannot usually leave the mover fitted to the caravan when driving, so you need to be able to easily connect and disconnect it, lift it and stow it.
An axle mover will always out perform an A frame mover on significant gradients and rough or poor surfaces, such as those you might find on caravan sites.