Police arrested William Tinkler, 60, charging him with misdemeanor animal cruelty.
Elgin officials were called to the home in the 200-block of Villa Street after someone doing work nearby spotted the van with the animals inside.
Animal workers found around 12 live cats inside the home. Those that could be caught were taken to a vet for a wellbeing check. The city will return for the others.
Police say they don't know what caused the cats to die or how long they've been dead.
"We just thought it was maybe he didn't take the cats out," said a neighbor. "He was here by himself and he was sick. He was a sick guy, and I don't know, I always seen him working out here in the yard and I just thought he loved animals."
Officials say Tinkler admits to placing the animal carcasses inside the van.
The Elgin buildings department has put a sticker on the home, marking it condemned.