Regarding the Chevrolet and Corvette signs around Road America (and the Corvette Bridge between Turns 5 and 6), they’ve been good supporters of Road America for decades. Regarding Camaros and Corvettes in the IMSA series, all of Chevy’s support seems to be going to the #3 and #4 Corvette Racing cars in the GTLM class; Corvette Racing is currently 2nd in the manufacturers’ standings:
https://corvetteracing.com.
GM has sponsored Camaros, Corvettes and Cadillacs in other IMSA classes in the past. I’m sure they think they’re getting more bang for the buck dumping big dollars in the NASCAR program for the Camaros and Chevy engine program in IndyCar.