Hello Smiley
As you can seem most of the other replies are suggesting units that are permanently fitted to your caravan, and drive the main wheels of the caravan. These are without doubt the most capable units, as they can help to position the caravan on most sites and storage compounds, copping with slopes and uneven ground. The two down sides are the cost and the reductions of caravan payload.
If you are looking to only position your caravan at home, on hard level ground, then there are other movers. These typically fit to the hitch or the front A frame, and have their own battery. They are fine on level hard ground, but they can easily loose grip on slopes or on soft or gravely ground.
I have to say that you can usually find examples of the A frame movers in secondhand adverts - the reason is that many owners decide that the mover is not as universally capable or safe and are often upgrading to a main wheel drive system.