Good question.
We just purchased our first RV too. So, knowing that the "reader" in the toll booths are above / in front of the vehicle, I think I might be inclined to mount it at the base of the windshield and only have it there when passing through a booth.
NYS suggests mounting them behind the rear-view, so as not to impede line of sight. But, if it's only down near the bottom when needed, I think we should be all set.
I be attentive to any other suggestions.