If you mount it either high up in the windshield where it wouldn't obstruct your view of cross traffic or pedestrians, or low enough in your windshield that it only obstructs your view of your own hood you should be fine from the obstruction laws.
Since you have a 5er you won't really be using your center mounted rear view mirror. I might suggest you remove the mirror for this trip and place it there, or else lower the mirror all the way and attempt to mount it above the mirror.
My wife has a Garmin 5" GPS that has a ball mount system. It came with a suction cup mount. I was more concerned with theft from my vehicle by having the tell tale suction cup ring on my windshield. We found a weighted mount with the ball mount meant for the GPS and can either place it on the dashboard or the center console. I prefer that method when we use the GPS.
2015 Ram CTD
2015 Jayco 29QBS