Mary,
Sorry to hear you have mice in your caravan.
First of all clean up all the droppings and make sure there are no mice left in the caravan.
Look under the bunks and in cupboards and cover all the air holes with very fine metal mesh. Make sure all the edges of the mesh are sealed.
Where pipes, gas water or electric, go through the floor of the caravan, make sure the holes are fully sealed with mastic round the sides of the pipes. Don't forget the surround of the air inlet for the fire under the caravan.
Open the gas locker and check the bulkhead is sealed all the way round the edges with mastic. The slightest gap round the edges means mice can enter the caravan from the gas locker.
Double check underneath the caravan for any holes you may have missed.
It is a list of jobs but the only way to be sure no more mice enter.
Hamer