In the first instance it depends on if you tow or not as if you do and want sensors on the camper/van you will either need a range extender or not. After 20 years of testing and using TPMS we have chosen one from LSM for the D4 and Ultimate as it does not need a range extender and given they operate in the mining industry the external sensors can and have taken a pounding. Info can be found at this link;
Tyre Pressure / Temperature Monitoring Systems - Tyre Pressure / Temp Monitoring Systems
If you don't tow then the option widen we have used the Schraeder Air Aware and Sensatyre systems extensively and both are very good. Albeit the sensatyre needs a range extender if you want to tow so an additional cost.
Other brands have already been mentioned but our experience with several of them is that the external sensors don't handle the 20.000km or more we do off bitumen each year but I doubt anyone on this forum has that need. As well we have seen that each sensor is wheel specific so if it fails you need another of the same sort, vis LHF etc.
We like the LSM version as we carry a couple of spare sensors and they can be programmed to the wheel required rather than having to carry four etc.
One other comment is price is rarely a measure of accuracy and reliabilty
Rob