THe 123 chassis manual devotes 84 pages to the self leveling suspension...

If you read and follow that advise in those threads from the search.... You are likely to find that some MUCH SIMPLER and EASIER to fix item in your suspension is at fault...like a stuck or broken level control arm...

If the event that is not the case.... it is possible that just buying new nitrogen pressure cylinders would be easier than actually testing them.

If not, then you may need to acquire a 123 chassis manual for repairing them. It is not always feasible , or safe, for some items, to rely only on what is available on a forum....

These items are under pressure, with possibility of oil mixed into whatever might be sprayed into your eyes..... wear appropriate eye protection when dealing with it.