Here is the link to the factory specifications for hitch ball height for various model and year Airstreams. http://www.airstream.com/files/libra...c5c1005fbf.pdf
Like the previous poster said the hitch height to make the trailer level will drop, if the axles have taken a set. The measurements given are a good starting point. You can actually take a measurement of your unit fairly easily, if you can park it on a dead level parking lot unattached from the truck.