In Tennessee (other states, too), it is not illegal to have a trailer hitch ball in front of your license plate. They're recently begun erecting ALPRs everywhere...

...so I have a trailer hitch ball hung entirely across my plate — not considered "obstructing view" de jure, but YMMV (depending on officer).

Tennessee does not issue license plates for most trailers, either, so you can easily & even more legally conceal your license plate when towing.

Anything else that obstructs the view is illegal (including bicycle racks, leaves, dirt, lenses). But not trailers & hitch balls.