Just returned home from a wonderful 9,200 mile trip around the USA in my recently purchased '07 XJR. This is a marvelous travelling car!!!

Unfortunately, during the last 500 miles her rear suspension sagged down completely, with "Vehicle Too Low" and "Air Suspension Failure" messages in the speedo display. The front suspension seems to be OK.

So, given that the rear axle sank uniformly, and the front axle seemed to maintain original ride height, can you suggest a troubleshooting sequence?

My plan is to take the car off the road and replace all the suspension bushings, etc. I would like to properly diagnose the air suspension issue before I tear into things

BTW, my desire is to maintain the XJR CATS air suspension. One of the front struts has already been replaced (by a PO) with an Arnott unit. So, I'm looking for a way to return all struts to OE XJR units (without breaking the bank). Appreciate any ideas.

OE Equipment? Breaking the bank os relative to the individual, but Good luck.
How many miles on your XJR?
I experienced the same after a road trip. Fortunately, I was 5 miles from home, when my front did the same. One of the OE fronts leaked/gave up/bit the dust @ 69k miles. The other front had already been replaced with a rebuilt unit, so I replaced all 4 with Arnott Sports, along with the bushings. $1800+ Government fees for the Arnotts. Make sure to research posts before setting off on the bushing replacements. A host of cost saving wisdom is available. I saved hundreds in parts and did the replacements myself. Also, if you choose to have bushings pressed In the arms vs replacing the entire arm, make sure you scribe the old bushing on both sides, before removal. Trust me, this will ensure the new bushing is seated exactly where it should be. Tolerances are very tight.

Too many places it might be leaking to give you a definitive answer. Valve body under the spare tire, fittings on the struts, the struts themselves may have a structural failure in the air bag. You need a scan too that can read the Air Suspension midule before you can even guess what's going on. Such a tool can be had for less than 200 dollars, but you cannot troubleshoot the air suspension without reading the codes it stores.