As Monkey says, it is extremely unlikely that you will need to remove your aerial as there are always high vehicles together on the ferries.
However, should you need to remove it is quite a simple operation. You should have been given a small allen key with your aerial, plus a small plastic cap. Use the allen key to loosen two grub screws around the base of the aerial assembly. The are located around the circular piece that is permanently fixed to the caravan. When the grub screws are released the aerial is then free to lift out. Before you lift it out, disconnect the cable that plugs into the input socket of the aerial amplifier located in the locker in the caravan. The cable will then pull out as you lift the aerial. Use the plastic cap to cover the hole that is left in the base plate on top the van.