The present invention relates to a trailer drive unit in the form of a caravan mover comprising a motor 16 (drive means/drive device), a head 14 housing a roller 20 which are linked together by a gear mechanism 22. Specifically, the present invention provides a support and drive transmission system 22 located centrally on the roller 20 with the roller 20 being provided by a first lateral portion/roller 19 and a second lateral portion/roller 21. In this arrangement, the length of the lateral roller 10 portions 19, 21 are individually shorter than a single roller such that the torque created in the central drive transmission portion is significantly less.