As Dave says, you need to account for dips and slopes and more. Our 19' was approximately 20'2" in length with the spare tire. (We use 9 1/2' as a minimum clearance for our 21', for 21' owners with A/C.) I would add a few inches to the measurements you are given, which you seem to be doing. Looks as if you are on the right track.
Cathy. Floating Cloud
"Live in the sunshine, swim the sea, drink the wild air.... "