It's a bit hard to find. For mine:
Looking directly into the unit with the cover off, it was off to the left. Here's a picture of what it looks like. It's that small white button.
If the image doesn't open, right click on it and open in a new tab to see.
2015 Jayco Precept 31UL
Tow: 2014 Ford Fusion SE HYBRID
Blue Ox BX7420 Avail Towbar
SMI Duo "Stay in Play" Braking
Barley, our Golden Retriever, on board and Buster in spirit!