Make a solution of Dawn and water in spray bottle or better yet a garden sprayer. Hose down the air system feeding the bags and the bags themselves. Look for bubbles. Please crib or support the coach before getting under there.
If the leak is not then obvious and you suspect the valving that feeds the air bags, then you can make a quick and dirty isolation test device.
Essentially, temporarily disconnect one of the feed lines to an air bag and plumb in a device that consists of airfitting to match, pressure gauge, quarter turn isolation valve, pressure gauge, air fitting to match. Air up the coach, and then close the valve. Watch the gauges over a couple of days. That will tell you if the leak is in the air bags or the system that feeds the airbags.
If the isolation device isn't obvious, PM me, and I'll send you a sketch.
You can use the isolation device in other parts of the air system until you narrow down the leak area. The idea, just like electrical troubleshooting, is that you want to keep cutting the problem in half until you find it.
Is this some work? Yes. but you asked how do you find the leak.
You can spend a lot of money guessing what is leaking and throwing parts at it.
Richard and Rhonda
1999 Newell
Subaru Outback toad