measure....measure.....measure
The literature has no clue how a trailer will "sit" on your tow vehicle...it is a guestimate, based upon level towing, in a perfect world. May not include ad-ons installed by a dealer.
Do as others have said: Straight-edge from highest point to the ground, while trailer is attached to tow vehicle sitting on level ground. My personal opinion is to add an inch or two...just in case.
Write it down, know it....watch for signs that say CLEARANCE. If the clearance sign is less than or equal to the number you wrote down. GOOD. If their number is less than you number SSSSSSSSSSSSTTTTTTTTOOOOOOOOOOOOOPPPPPPPPPPPP