I can't directly answer your question but our 351 RLTS has a covered belly. There are holes about 2' x 2' that are covered with squares of the plastic and screwed in place. Access to my manual retracts is via these "hatches". I believe the connection you are looking for will be close to the motor for the drive mech. My motors (and manual retract) are close to the frame on the opposite side from the slide. My manual retract requires a ratchet wrench.
Have someone activate a slide while you are outside, listen for the motor and look underneath for some kind of access. If the crank is for the slide I'd expect a hole low on the side or in the frame for access. Are you sure the crank is for the slide and not the landing gear and/or spare tire, my 2 cranks are for those.
My use manual is also vague on where/how and on our 2nd outing I hadda figure it out.
2013 F-350 CC SB 2WD 6.7PS
2013 Eagle Premier 351 RLTS
-SOLD- 2012 X23B
-SOLD- 2003 Ford Expedition 5.4, Bilstein shocks