Had a look to see where my blue wire went up front on fire wall fusing and couldn't pin point where it is, my coach is gas so is not the same as yours anyway, but there still is no indication of where it winds up. Everything is well marked but nothing to indicate where harness wiring for the trailer plug is terminated.
Not sure who places the trailer plug in if its Newmar or the chassis MFG.

If you give a call to Newmar maybe they can tell you or who installs the plug and where to go from here,sorry.