I also have a 2015 REQS and did replace my rear sensor a while back. On mine, I had to drop the spare tire and cut the plastic underbelly in the middle of the trailer for access to the trailer’s cross beam. My sensor was mounted on a small block of wood screwed into the “C” channel of the frame.
Lippert told me this was NOT their recommended method of mounting for the RV manufacturers but that Jayco (and others) did not always follow their recommendations. They recommend the sensor be mounted on a metal bracket welded to the frame. Vibration is the main enemy here.
Since I did not have welding skills nor equipment, I used a piece of angle iron bolted to the frame using Loc-Tite to mount the replacement sensor. So far, (fingers crossed) my mounting has held tight.
Just another “fix” for Jayco’s version of quality versus mine. Good luck.
2015 Pinnacle REQS
2016 Ford F-350 Platinum Dually