If you want to monitor the tyres on your trailer / caravan only then you would need either a TD-2700F-X02 (2) Wheel TPMS Kit or TD-2700F-X04 (4) Wheel TPMS Kit. If you wanted to monitor your 4x4 / passengers vehicle and trailer / caravan then you would need either a TD-2700F-X06 (6) Wheel TPMS Kit or TD-2700F-X08 (8) Wheel TPMS Kit.
With these systems, you would need to mount the relay module underneath the rear of you vehicle. If you tow multiple trailers and you don't want to switch sensors between your different trailers then we would recommend a TD-2700F-X04 (4) Wheel TPMS kit for your vehicle and either the TD-RY2700F-X02 (2) Wheel Addon Trailer kit or TD-RY2700F-X04 (4) Wheel Addon Trailer kit for your trailers.