According to the ZF 15 technical manual, the ZF15 gearbox
operating temperature should range between 122 and 176 degrees F with occasional forays as high as 212 degrees.
Maretron makes a NMEA 2000
temperature module , model TMP100, which interfaces various temp probes into a NMEA 2000
bus. The most simple of the probes is placed under a bolt head
and will give you the case temperature at that point. Other probes, such as liquid sensors, might be more accurate but involve trickier installations, such as drain or fill plugs.
Have fun and good luck.