My understanding is that the air chuck is connected to the air tank that supplies air for your suspension system and your brakes. It is an outlet chuck that lets you use the air in the tank to pump up tires, etc. I had one on my Mandalay and used it many times to pump up tires. I also have one on my new Excursion and got the same explanation that you did when I picked it up from the dealer. I fully intend to use it to top off my tires when needed.

We had such a connection in our previous RV.  It works very well for topping off tires, but it can be very loud when it gets disconnected - good idea to put a towell over the connection when you disconnedt it.