http://www.iowadot.gov/mvd/omve/truckguide.pdf On page 6-9 has some input.
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http://www.iowadot.gov/mvd/ods/cdl/cdlmanual.pdf Page 1.1 says if you are required to have a CDL.
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http://coolice.legis.iowa.gov/Cool-ICE/default.asp?category=billinfo&service=IowaCode&ga=83&input=326#326.1 Under definitions and para 326.2 describes a commercial vehicle. Reads the same as fed definitions.
Your situation as you described is a common issue especially with a officer that has never been through his states commercial trucking training program. What is odd that the officer was a captain..