My memory is pretty rusty on the details of that system, but I do remember that each bag is controlled by a solenoid (or am I thinking of my wife's Expedition?!) .... anyway, jack the car up, use jack stands capable of handling it's weight & trace the air lines coming out of the bags. It's curious that BOTH rear bags are deflating - make sure it's not just one side "dragging" the other side down with it - that will make diagnosis a bit easier.
Actually.....
Do this first -
1) Park the vehicle at normal ride height, maybe when you've arrived home from work & don't plan to use it again till the next day. Disable the air system (cant remember if it's a switch, fuse, or??)
2) Measure & record height from the concrete to a specific place on all wheel wells - use some masking tape on each fender arch and record each number there so you don't confuse them. They will be different, it's of no consequence at this point.
3) The next day, take the measurements again & compare.
Hopefully this will narrow down your issue & make the "hunt" more fruitful!