Here's the resource you want to use.
http://www.iowadot.gov/maps/msp/dlalphabetcounty.htmlNote that the PDF's in the "Color PDF" column will automatically color the gravel roads: Red, and green are dirt. Easy to pick a county and see the routes that you can string together. It gets a bit tricky if you want to cross county lines, but using multiple windows on your PC will help that.
NOTE: As these maps are DOT reference material, you can "mostly" trust them, but they are definitely NOT 100% accurate. They ARE the most accurate maps I have found for Iowa though. With over 70,000 miles of gravel/dirt roads, it isn't any wonder that the maps are not 100% accurate. Things are pretty fluid out there in terms of roads closing, being worked on, bridges out, etc.