Without a way to monitor the transmission oil temp, there is no real accurate way to measure the fluid level. However, without a leak, one would wonder why the fluid level would change. In addition, MB seems to actually more concerned about overfilling than underfilling, as the case is scavanged of oil to increase efficiency. But it is important to be certain the oil level is correct. You should also examine the color of the fluid to make sure it isn't cloudy looking, as a bad trans oil cooler will cause coolant to get into the ATF, which can lead to this problem (harsh shifting).
I would also ask if the correct ATF was used, the special trans oil MB sells specifically for the 722.6.