Well, I crawled under the rear of the coach this morning and found the Transmission Control Unit.
On my 2012 XC chassis, it is above the driver's side rear air spring mounted inside a metal box bolted to the frame rail....as depicted on my Freightliner drawing. (See Photo)
Unfortunately, it is nestled in a hornets nest of black and red wire loom and since I was not dressed for the occasion to dig deeper amongst the dirt and grime, I couldn't follow the wiring harness with my mechanics mirror or find anything in the general vicinity that looked like a grounding stud.
I will comnent that that TCU housing looks rather robust and if yours is or was the same, I'd be very surprised if it failed as indicated by the repair shop.
Again...if they used the same housing on your chassis. Your set-up may be totally different.....we'll see.
Good luck getting connected with (Daimler) DTNA Freightliner for assistance.
Let us know what you find out.