Hello William,
The replacement jockey wheel movers are fine as long as you are using them on flat hard ground (i.e. concrete or tarmac etc)
As soon as you begin to encounter a slope or obstacle, the down force on the driving wheel may not be sufficient to maintain drive, or more importantly braking. - Consider if the van starts to roll down a hill, if you cant stop it yourself, then the jockey wheel mover is unlikely to be able to either.
There is another consideration. The chassis manufacturers never intended the jockey wheel location to accept the loading and thrust of a mover, and it is quite possible to twist the side member where the jockey attachment is fitted. This is not covered by warranty.
In terms of operation, whilst you are using the handles, you cannot see where the rear corners of the caravan are going, so there is a real chance of a slow speed collision when manoeuvring in tight spaces.
The device cannot be left on the caravan whilst towing as it will reduce the floor clearance, and may foul on the body of the car when turning tight corners.
There is the alternative type of hitch-mounted movers, which have similar safety issues on slopes and poor surfaces.
The other alternative is the type that drives the main wheels. These are far more capable of controlling the caravan, plus most offer a remote control that allows you to watch the corners whilst moving the van. These are available where ever the van is.
The general consensus in the caravanning fraternity is give the A frame movers a miss, and go straight to a main wheel system. See the number posts about this subject on this forum and also look at the number of 'A' frame movers for sale second-hand - it must tell you something.