Presently there is a lot of controversy about the remote sensors. Some use the ABS system or wheel speed monitoring. An under inflated tire rolls slower(or faster) than a correctly inflated one and this info is read and passed to a monitor the driver can see. The Dept Of Transportation has issued some test results and some of them don't work as advertised.
As I almost never go more than a hundred miles without stopping I went with a different type. They are visible sensors that replace the valve stem cap. They come in various pressure settings. I got four 35 pound monitors for the tow vehicle and two 50 pound monitors for the TM. If the pressure remains within the set tolerances, you see green on the monitors, if it is down a little, you see Yellow and if it is dangerously low, you see red. A quick walk around the rig gives you all your tire info.
My theory is that most leakage is slow, so a visual monitor will suffice. A sudden loss of pressure, such as a blowout, will make itself known without a sensor in the drivers compartment.
Dave