The 2015 did not come with an OEM receiver hitch or a 2-5/15" ball. It came with a solid 2" ball shank that is removable and there is a separate 2" box receiver that is only rated for bicycle rack use. A 2-5/16 ball shank was subsequently developed and released to resolve the AS ball size incompatibility.
So, step one is to find out exactly what you have. Do not panic. All is not lost. It's just that you need to fill in the info that BMW does not provide. JCL and others have offered much. Search for BMW X5 info on the tow vehicle and hitch sub-forums here on AirForums. Search hitch info on the Bimmerfest forum too. The back story is there if you look for the threads.
Not equipped with the air suspension is not a problem. With the air suspension, you have to weigh the rig at a CAT scale to verify proper weight distribution and balance. Air suspension levels the X5. It does not transfer weight. You need the WDH for that function. Do not tow an AS on the ball.
Congrats on the new to you X5. It is a great AS tow vehicle. We have the wimpy little 2015 x35i and are very happy towing our FC23FB.
Note, the FC20 is a great coach and has a dedicated thread to pitch it's value. The 22Sport is a little 23FB and has many advantages. The smaller size is great for a lot of applications. The 19 has a following too, but most folks buy a 25, as it is the most popular. Final choice is a matter of finding the floor plan that works best for you. Understand all before you sign.
Hope to see you down the road with a smile.
Pat