Simple answer is that it's located near/at the slide motor, but finding it may not be as simple. On some models there will be a place identified by a sticker on the outside of the trailer, some are accessed from within the TT, but on many units it will be located underneath the TT.
In my case there is a small access panel that I can remove from my enclosed underbelly that provides access to the motor where I can attach a crank (or wrench) to manually move the slide.
My panel opening:
If you have trouble accessing the slide motor, contact your dealer or Jayco to confirm the manual location area.