The self leveling is ajustable and the car will sag badly if it is deflated. Measure the height of the rear wheel arches and then add a bunch of weight (a couple people in the rear will do). Drive the car a couple miles and remeasure the arch height in the same place with the people still in the rear. See if it levels. If it doesn't it probably is deflated.
Bosch Master, ASE Master, L1
33 years MB technician