Doniphan, NE
Pyatts, Illinois.
Denver, Colorado is the approximate halfway point.
A great place to stop for a halfway mark between Waterloo, Iowa and Chandler, Arizona would be Guymon, Oklahoma. There are quite a few good hotels and restaurants in the surrounding area of Guymon.
Smeltertown, Colorado is exactly halfway. You could try Denver if you want a bigger city.
The halfway point is Conover, Iowa.
The halfway point between Davenport Iowa and Detroit Michigan is in New Buffalo, Michigan.
West Union, Iowa is the halfway point.
The halfway point is Chester, Iowa.
Kansas
West Des Moines, Iowa
The town that marks the exact halfway point is actually Mack, Colorado but a better choice might be Grand Junction, Colorado which is about 23 miles from the exact midpoint.
The elevation in Waterloo, Iowa is 883 feet above sea level.