If by undiscovered, you mean places I have never visited and can't accurately vouch for whether the tenderloins are homemade, then here is a list for Iowa between I-80 and HWY 20 East of Des Moines/M-Town. I would call ahead for hours and to verify that they make tenderloins themselves before making plans.
Mid-Town Family Restaurant Iowa City
Rookies Sports Bar in North Liberty (an award winnner)
Flying Wienie (probably frozen tenderloin but still roadfood-worthy establishment) Cedar Rapids
Hamburg Inn Iowa City (pretty obvious choice)
Charlies 11 St. Precinct Bar & Grille Davenport (a mystery spot for me)
Highway 63 Diner Waterloo http://www.landaudiners.com/highway63.htm
Tic Toc Cedar Rapids http://www.takeoutnetwork.com/tictoc/
Slaby's near Davenport in Walcott
Bluegrass Cafe Tama
Kalme's in St. Donatus (its the luxemburger town, all 5 builings in it)
Niland Cafe in Colo has Roadfood written all over it, its a "one-stop" vintage gas staion, diner, and motel renovated with funds from locals and county redevelopment money. It has museum-style dioramas with Lincoln Highway artifacts inside.