I had a bailey caravan where I had the same problem directional roof aerial not working.
Always switch the amplifier on should get a red light to say the 12 volt supply to the amplifier is working.
Now I tested my aerial by removing the input lead from the aerial lead that goes into the amplifier, connected it to a directional signal device(which you can purchase from a Good dealer) turning the aerial around found 4 green lights which proved the aerial was okay.
The fault was behind the aerial connection socket inside the caravan , the screws holding the sockets remove the two plastic bungs which cover the holding screws, I gently pulled the socket cover away from the socket , what was exposed was that the middle copper wire of the aerial had come away from the internal connection, managed to get a bit more TV aerial through the socket to enable me to reconnect the copper wire , Switched the 12 volt supply back on, tested the aerial via the internal reconnected socket to the wall , and got the 4 green lights connected telly to socket picture perfect.
Trust this is of some help Royston