Welcome to JOF
I assume you are using a Weight Distribution Hitch (WDH)??
I don't know the actual height to the top of the X23B "ball coupler", but 21" sounds about right. I would contact Jayco Customer Support directly to confirm. (574-825-5861)
On some of the bigger Jay Flights I recall hearing a ball coupler height of about 23 3/4", but they may sit higher because of the full slides and tire size.
It is possible that you may require a longer draw bar that will allow you to lower the WDH ball mount to get the required ball height.
: I found an older thread, and per Jayco the ball coupler height is 20" on a 2009 23B, so the 21" may be correct on your 2015 X23B.
Reference thread: http://www.jaycoowners.com/forums/sh...er+ball+height