Hi! I have a 2011 GL450 and replaced all the air shocks. I needed to as my symptoms were like yours, starting in the front. I couldn't find the leak anywhere for a long time. Finally I sprayed soapy water on the air bag and as the vehicle was lowering and the airbag was going through the process a leak suddenly appeared. It was only visible when at a certain height. At other heights when the airbag was in other orientations the leak was too small to see. I suggest starting with soapy water on the airbags and watching all around them as the vehicle raises and lowers so you can see the airbags in all of their positions.
If you need to get to the compressor, you'll have to take off the front right wheel well to access that area. I hope this helps someone else with this problem.
