As a police officer, I can only speak for my state, California. Here, the law would not only require you to re-locate the license plate, but add a light to illuminate it at night. While the light provision would rarely be enforced, the unobstructed view of the plate often is. I would venture to say that most states have similar laws.
2005 Fleetwood Providence 39J
CAT C7 350, MP-8 Programmer
My wife does all the driving - I just hold the wheel...