Hello Lisa,
When I first saw your post I thought you were looking for a bike rack to use whilst not towing, but Phils answer prompted me to point this out.
Your cars towball limit is not just for teh tow ball but for any load that is carried by the towbar support system.
I'm sure you must have seen similar topics on this forum about towhitch mounted bike racks. But just in case you haven't, then please be aware that the cars nose load limit will have to accomodate both the caravan and the rack with the weight of the bikes - so if you have a car tow ball vertical load limit of 75Kg, and your rack and bikes weight 25Kg, then that only leaves you with 50Kg for your caravans nose load.
One way round this is to do what my BiL did which was to make a bike rack the fitted to the A frame of the caravan. This meant that he had to retrim his caravan to get the nose load right but the weight of the rack and bikes formed part of the caravans actual nose load, rather than detracting from it.