Use enough sensors for every monitored trailer tire.
READING GUIDETIRE SAFETY
Travel Trailer TPMS Basics
What a trailer tire-pressure monitoring system can tell you and what it cannot replace.
A trailer TPMS gives you pressure and temperature information while travelling. It is an early-warning tool, not a replacement for inspecting tires, checking cold pressure and understanding tire age/load limits.
What matters in practice
Metal vs rubber valve stems can affect sensor suitability.
Longer rigs may benefit from a signal repeater depending on the system and tow vehicle.