Like noted before-sounds like a air leak in the system...there are a few types of leaks:
1. leak at either of the 3 valve blocks - easy test is to pull your suspension fuse before you park it at night - check how the car stance is in the morning (if a single axle is on the ground - ie if the rear is raised but the front axle is on the ground then you most likely have a leak at either the front valve block or both of the front air bags). Valve blocks are super simple to repair - just need to get an o-ring kit from britishpartsofutah or similar site and rebuild them - hardest part is just getting the valve out of the truck - front is straight forward - rear is quite a bit tighter but still doable - middle is super simple but rarely has issues.
2. if there is a leak at any individual air bag - and you perform the fuse test and if only a single wheel is on the bumps then that airspring needs to be replaced (I'd just replace the entire shock and spring combo - and I'd probably do the whole axle at the same time because when one air spring goes likely the other is just about ready to go too. Arnot makes a great factory replacement at a fraction of the cost - I've been running them for 35K miles now and still feel like the day I put them in so I expect them to last at least another 50K.
3. if the middle valve block is leaking (pretty rare but could happen) then just replace the o-rings and go with it - this could be indicative of the entire car on the bumps. I have had my valve blocks leak back into the actual tank as well and it makes it a little harder to tell what's going on - still only o-rings but acts similar to a single valve block leak.
note if you don't pull the suspension fuse the truck will automatically try to level itself by purging air throughout the night until all corners are on the bumpstops so by pulling the fuse it'll stop that and the natural leak will be exposed...a good way to verify if you have a corner bad is to mix some soap and water and spray down each connection and look for bubbles too. If the compressor is going it'll just take longer and longer to refill the truck until the compressor over-heats and will eventually fail due to excessive duty cycling...
RSW put out a decent air suspension diagnostic video...probably better then my explanations above as I'm on some heavy pain killers after I broke my leg so it may not make fluid sense...I did use this video when I started trouble shooting mine so may be of some help...there's also a video of removing each of the valve block and the compressor if you need it - those are a bit easier to find if memory serves. good luck!