Hello Andy,
There have been a number of reported difficulties with tow-hitch mounted bike carriers when towing.
Firstly, there is the physical size of the bikes that can interfere with the caravan when taking bend or reversing.
And secondly, as the tow hitch on your car has a limited nose load capacity of 75Kg, you must appreciate that to keep within that limit you must reduce the caravan nose load down force by the weight of the cycles and the rack. This could leave you with only 50Kg available for the caravan.
Reducing the nose load of the caravan is not recommended, as it can adversely affect its towing characteristics.
Please check through the forum search system, but you will find that most cycling caravanners tend to opt for roof mounted cycle carriers, or securing the cycles inside the caravan.