As others have mentioned, rvparkreviews.com is a great resource. CGs are listed by city within the state. There's a wealth of them in Nebraska, not quite so many in Iowa. Over the years, on many trips across the plains, we have been to the following and found them at least acceptable for an overnight, if not enjoyable. (Asterisks are for the ones we especially liked.) Bear in mind that we like state parks, large sites, and some amount of privacy, and that we don't necessarily need or want FHUs.
*Des Moines West KOA, Adel IA
*Ft. Kearney St Rec Area, Kearny NE
*Kellogg RV Park, Kellogg IA (funky little place but we like it - nice little mom & pop burger joint, gas station & RV park)
*Grand Island KOA, Doniphan NE - good pizza, delivered to your site
*Mormon Island SRA, Grand Island/Doniphan NE - note, large rigs may have a bit of trouble fitting in many sites here
Newton KOA, Newton IA
Camp-a-Way, Lincoln NE
*Holiday RV Park, North Platte NE (nice & shady)
*Prairie Oasis RV Park, Henderson NE
Kearney RV Park, Kearney NE
**Robidoux RV Park, Gering NE (off US 26 - the "scenic" route alternative to I-80 west of Ogallala NE)
2020 Toyota Tundra CrewMax 5.7L V8 w/ tow pkg, Equal-i-zer
2020 Lance 2375