Sounds like you have an air leak(s).
To safely check you'll need to be able to crawl under your rig not relying on the air system or hydraulic jacks if you have them.
I built a set of ramps using 2X10" lumber cut in 48, 42, 36, 30" lengths. Screwed together to form a ramp, I added a stop block on the top piece to prevent me from driving off. I built 6 of these, one for each tire.
I then used a spray bottle of soapy water and checked all my fitting, valves, connections, and check valves.
I ended up replacing all my Pressure Protection Valves because they were leaking badly. I also replaced any fitting that I had to excercise to get the tubing free, and other check valves because they had internal rubber components that could fail with age.
None of my leveling valves were leaking. Here is a link I did on replacing the PPV's. https://www.irv2.com/forums/f115/pre...ng-562359.html
If you have automatic air leveling there is an air manifold that is known to leak, they call it a 6pack. This might be your problem and you can rebuild these.