Just had the very same issue on my 2021 GMC 3500 after towing my travel trailer.
The local GMC Dealer traced it to the 30 AMP fuse in the block under the hood.
Fuse part is 22917202 (large 30 AMP fuse)
Fuse circuit is No 63 (TRLR BATT)
Kicker Stereo works fine now but let’s see what happens when I hook up my trailer next time?