What you describe does exist in the aftermarket. I have one made by Hopkins (Hoppy) on our trailer. It is designed large so it is visible from the drivers seat via the rear view mirror when parking. I love the idea/theory, but in practice have found that the ball is "sticky" and does not move unless you change the angle several degrees or tap the level. I have jacked one side high enough to remove the wheel/tire for maintenance and had the ball not budge.
It is liquid filled and I have considered mounting it upside down so I can try just using the air bubble instead. I have a feeling it would be far more precise, but probably not as easily seen through the mirror.
'04 GMC Sierra 2500HD, CCSB, D/A, MaxBrake, B&W Companion, Retrax Pro, AR Power Steps
2013 Rockwood 8289WS w/Diamond pkg
1969 Camaro RS/SS Convertible
Dad's old 1968 Empi Imp (recently recovered from under 21 years worth of boxes/dust)