if swapping the antenna didn't fix it, it could be the satellite receiver unit itself, or the wire leading to the antenna.
btw, when I drive in florida I notice that when next to big-rig the signal could cut out rarely/briefly.
a possible way to prove there is problem with vehicle sat system is to use another receiver, and position the other receiver's antenna next to the benz sat antenna. you can do this with a single subscription because the signal problem will be evident even on channel 184, the no-subscription channel.
also you may be able to demonstrate this in the benz dealer's parking lot. drive around until you find a spot where the problem happens. then drive another benz from the lot next to your car. crank both radios, and point out to service tech when your radio goes off and the other does this. swap the cars positions, and then demonstrate the issue again, to prove that a 10 foot offset in position is not the cause.