I agree I have a 1998 Ford Expedition EB I had the same problem the rear of my truck started sagging after sitting overnight. I didn't worry about it to much which ended up being a big mistake. The hole's in the bags ended up getting worse and before long not only did I need new bags for my expo but needed a new compressor and dryer. That's what the mechanic told me, and what it was going to cost a ton. Well I searched around and heard about strutmasters they do conversion kits it takes off the air system and put on shocks and springs. I have really enjoyed the ride and all. Hope this was some help the website is strutmasters.com good luck.
You should find where is leaking from and fix it before your compressor burn out from running all the time.