The OEM TPMS vary by manufacturer and even model. Some models, usually lower end, do not use a transmitter/receiver setup at all, instead relying on speed differences and other parameters from wheel to wheel as sensed by the ABS and other sensors. Designing and producing a remote monitor that could be used with a variety of different frequencies and coding methods for those models that do use a radio based system would be costly, and have a limited market. Buying four extra sensors for an existing RV TPMS makes much more economic sense.