2011 Silverado braking vibrates during downhill

Yes, no one makes decent rotors anymore. Even the GM parts are made in China. They warp from heat almost immediately. The new-ish rotors on my Avalanche are okay until they get hot - like slowing down from highway speeds. Yours are vibrating because they're getting hot slowing the trailer down. I hate to say it's normal, but it sort of is these days. At least on GM products.