In the US, a warranty issue would have to be proven to be caused by your modification to void coverage. Do what you like, but take heart that the Niro is approved in Europe for towing up to 750 kg trailers with brakes. That is the identical Niro to ours with respect to cooling systems and brakes. Original marketing from 2017 referred to a towing package (again in Europe, but that option has never been made available anywhere). Most just want a tow mount for a bicycle rack - can't even imagine what a warranty issue would come from that!
2018 Kia LX HEV Metal Stream with Advanced Tech