Los Angeles and Milwaukee are about 1,741 miles apart by road. Split the difference and you arrive near Denver, the city closest to the halfway point between them. That puts roughly 17 hr 24 min of driving on the Los Angeles side and 19 hr 8 min on the Milwaukee side — the fairest single meeting point among the cities near the middle.
Over this distance most people will fly rather than drive the whole way. Denver still makes a fair, central place for Los Angeles and Milwaukee to converge, splitting the travel instead of asking one side to cross the country.
If Denver doesn't have what you're after, Albuquerque and Oklahoma City are also close to the midpoint and worth a look — each keeps the drive reasonably balanced between Los Angeles and Milwaukee.
City descriptions adapted from Wikipedia (CC BY-SA); photos via Wikimedia Commons, credited above.