Tyre pressure monitors are great, we have a set of 8 Sensa brand which are mounted on the rim inside the tyre, with a huge stainless steel hose clamp. They are well protected and each sensor is suppose to have about 5 year battery life. Once fitted they are great, a bit of a pain to get fitted, the tyre is removed and clamp mounted on rim and then tyre balanced, but once on they just work. I suppose when we need to replace the tyres on the van we will need new sensors as the batteries may be at the end of their life.
We have a single axel van so we got sensors for both spares which we swap with van tyres each 10,000K the receiver display shows the 8 tyres, sometimes the spares on the rear of the van do not show, but eventually they show up. The 6 tyres in contact with the ground are displayed nearly all the time, sometimes one on the van disappears, but we have the receiver antenna mounted inside on our cargo barrier. It probably should be mounted outside.
We bought our set from Hannibal Safari in Brisbane and the technical support is excellent. They are not cheap, but once set up work well and alarms can be set for different pressures and temperatures by the user, if a sudden drop in pressure occurs they sound an alarm and turn on a light. So far we have not had a serious problem with our tyres, so no alarm, but we check the pressures each time we start up and after a about ½ hr of driving. The sensors also detect temperature which is measured from the rim, so can give and indication of brake or bearing issues, the temp is really interesting when you come down a steep slope like Talbingo Mtn in the Snowies. The last time we came down it, we got a warning light, but no alarm as the rear brakes on the tow got a bit warm, the temp got to one of our preset triggers, so a light came on. so wound the brakes up on the van and all ok.
https://mr4x4.com.au/gear-install-sensatyre/